Split off -core module.
Change-Id: I64d3c195db94e92da44c7e4971f5e85991ac30c8
diff --git a/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/LabelProvider.java b/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/LabelProvider.java
new file mode 100644
index 0000000..8cf87db
--- /dev/null
+++ b/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/LabelProvider.java
@@ -0,0 +1,49 @@
+package eu.mulk.quarkus.googlecloud.jsonlogging;
+
+import java.util.Collection;
+
+/**
+ * A user-supplied provider for {@link Label}s.
+ *
+ * <p>Instances of this interface that are registered with the {@link Formatter} are applied to each
+ * log entry that is logged.
+ *
+ * <p>If you are using the Quarkus extension, any CDI beans registered under this interface are
+ * registered automatically.
+ *
+ * <p><strong>Example:</strong>
+ *
+ * <pre>{@code
+ * @Singleton
+ * @Unremovable
+ * public final class RequestIdLabelProvider implements LabelProvider {
+ *
+ * @Override
+ * public Collection<Label> getLabels() {
+ * return List.of(Label.of("requestId", RequestContext.current().getRequestId()));
+ * }
+ * }
+ * }</pre>
+ *
+ * Result:
+ *
+ * <pre>{@code
+ * {
+ * "textPayload": "Request rejected: unauthorized.",
+ * "labels": {
+ * "requestId": "123"
+ * }
+ * }
+ * }</pre>
+ *
+ * @see StructuredParameterProvider
+ */
+public interface LabelProvider {
+
+ /**
+ * Provides a collection of {@link Label}s to add to each log entry that is logged.
+ *
+ * @return a collection of {@link Label}s to add to each log entry that is logged.
+ */
+ Collection<Label> getLabels();
+}