// Copyright 2014 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package prometheus provides metrics primitives to instrument code for
// monitoring. It also offers a registry for metrics. Sub-packages allow to
// expose the registered metrics via HTTP (package promhttp) or push them to a
// Pushgateway (package push).
//
// All exported functions and methods are safe to be used concurrently unless
//specified otherwise.
//
// A Basic Example
//
// As a starting point, a very basic usage example:
//
//    package main
//
//    import (
//    	"net/http"
//
//    	"github.com/prometheus/client_golang/prometheus"
//    	"github.com/prometheus/client_golang/prometheus/promhttp"
//    )
//
//    var (
//    	cpuTemp = prometheus.NewGauge(prometheus.GaugeOpts{
//    		Name: "cpu_temperature_celsius",
//    		Help: "Current temperature of the CPU.",
//    	})
//    	hdFailures = prometheus.NewCounterVec(
//    		prometheus.CounterOpts{
//    			Name: "hd_errors_total",
//    			Help: "Number of hard-disk errors.",
//    		},
//    		[]string{"device"},
//    	)
//    )
//
//    func init() {
//    	// Metrics have to be registered to be exposed:
//    	prometheus.MustRegister(cpuTemp)
//    	prometheus.MustRegister(hdFailures)
//    }
//
//    func main() {
//    	cpuTemp.Set(65.3)
//    	hdFailures.With(prometheus.Labels{"device":"/dev/sda"}).Inc()
//
//    	// The Handler function provides a default handler to expose metrics
//    	// via an HTTP server. "/metrics" is the usual endpoint for that.
//    	http.Handle("/metrics", promhttp.Handler())
//    	http.ListenAndServe(":8080", nil)
//    }
//
//
// This is a complete program that exports two metrics, a Gauge and a Counter,
// the latter with a label attached to turn it into a (one-dimensional) vector.
//
// Metrics
//
// The number of exported identifiers in this package might appear a bit
// overwhelming. Hovever, in addition to the basic plumbing shown in the example
// above, you only need to understand the different metric types and their
// vector versions for basic usage.
//
// Above, you have already touched the Counter and the Gauge. There are two more
// advanced metric types: the Summary and Histogram. A more thorough description
// of those four metric types can be found in the Prometheus docs:
// https://prometheus.io/docs/concepts/metric_types/
//
// A fifth "type" of metric is Untyped. It behaves like a Gauge, but signals the
// Prometheus server not to assume anything about its type.
//
// In addition to the fundamental metric types Gauge, Counter, Summary,
// Histogram, and Untyped, a very important part of the Prometheus data model is
// the partitioning of samples along dimensions called labels, which results in
// metric vectors. The fundamental types are GaugeVec, CounterVec, SummaryVec,
// HistogramVec, and UntypedVec.
//
// While only the fundamental metric types implement the Metric interface, both
// the metrics and their vector versions implement the Collector interface. A
// Collector manages the collection of a number of Metrics, but for convenience,
// a Metric can also “collect itself”. Note that Gauge, Counter, Summary,
// Histogram, and Untyped are interfaces themselves while GaugeVec, CounterVec,
// SummaryVec, HistogramVec, and UntypedVec are not.
//
// To create instances of Metrics and their vector versions, you need a suitable
// …Opts struct, i.e. GaugeOpts, CounterOpts, SummaryOpts,
// HistogramOpts, or UntypedOpts.
//
// Custom Collectors and constant Metrics
//
// While you could create your own implementations of Metric, most likely you
// will only ever implement the Collector interface on your own. At a first
// glance, a custom Collector seems handy to bundle Metrics for common
// registration (with the prime example of the different metric vectors above,
// which bundle all the metrics of the same name but with different labels).
//
// There is a more involved use case, too: If you already have metrics
// available, created outside of the Prometheus context, you don't need the
// interface of the various Metric types. You essentially want to mirror the
// existing numbers into Prometheus Metrics during collection. An own
// implementation of the Collector interface is perfect for that. You can create
// Metric instances “on the fly” using NewConstMetric, NewConstHistogram, and
// NewConstSummary (and their respective Must… versions). That will happen in
// the Collect method. The Describe method has to return separate Desc
// instances, representative of the “throw-away” metrics to be created
// later. NewDesc comes in handy to create those Desc instances.
//
// The Collector example illustrates the use case. You can also look at the
// source code of the processCollector (mirroring process metrics), the
// goCollector (mirroring Go metrics), or the expvarCollector (mirroring expvar
// metrics) as examples that are used in this package itself.
//
// If you just need to call a function to get a single float value to collect as
// a metric, GaugeFunc, CounterFunc, or UntypedFunc might be interesting
// shortcuts.
//
// Advanced Uses of the Registry
//
// While MustRegister is the by far most common way of registering a Collector,
// sometimes you might want to handle the errors the registration might
// cause. As suggested by the name, MustRegister panics if an error occurs. With
// the Register function, the error is returned and can be handled.
//
// An error is returned if the registered Collector is incompatible or
// inconsistent with already registered metrics. The registry aims for
// consistency of the collected metrics according to the Prometheus data
// model. Inconsistencies are ideally detected at registration time, not at
// collect time. The former will usually be detected at start-up time of a
// program, while the latter will only happen at scrape time, possibly not even
// on the first scrape if the inconsistency only becomes relevant later. That is
// the main reason why a Collector and a Metric have to describe themselves to
// the registry.
//
// So far, everything we did operated on the so-called default registry, as it
// can be found in the global DefaultRegistry variable. With NewRegistry, you
// can create a custom registry, or you can even implement the Registerer or
// Gatherer interfaces yourself. The methods Register and Unregister work in
// the same way on a custom registry as the global functions Register and
// Unregister on the default registry.
//
// There are a number of uses for custom registries: You can use registries
// with special properties, see NewPedanticRegistry. You can avoid global state,
// as it is imposed by the DefaultRegistry. You can use multiple registries at
// the same time to expose different metrics in different ways. You can use
// separate registries for testing purposes.
//
// Also note that the DefaultRegistry comes registered with a Collector for Go
// runtime metrics (via NewGoCollector) and a Collector for process metrics (via
// NewProcessCollector). With a custom registry, you are in control and decide
// yourself about the Collectors to register.
//
// HTTP Exposition
//
// The Registry implements the Gatherer interface. The caller of the Gather
// method can then expose the gathered metrics in some way. Usually, the metrics
// are served via HTTP on the /metrics endpoint. That's happening in the example
// above. The tools to expose metrics via HTTP are in the promhttp
// sub-package. (The top-level functions in the prometheus package are
// deprecated.)
//
// Pushing to the Pushgateway
//
// Function for pushing to the Pushgateway can be found in the push sub-package.
//
// Other Means of Exposition
//
// More ways of exposing metrics can easily be added. Sending metrics to
// Graphite would be an example that will soon be implemented.
package prometheus