Always include Collection tag in MongoDB observations.

Use default "none" collection name when command doesn't define a collection.

Signed-off-by: michaldo <michaldo@github.io>
Closes: #4994

Author: michaldo

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/observability/DefaultMongoHandlerObservationConvention.java

@@ -15,6 +15,7 @@
  */
 package org.springframework.data.mongodb.observability;
 
+import io.micrometer.common.KeyValue;
 import io.micrometer.common.KeyValues;
 
 import org.springframework.data.mongodb.observability.MongoObservation.LowCardinalityCommandKeyNames;
@@ -63,6 +64,8 @@ public KeyValues getLowCardinalityKeyValues(MongoHandlerContext context) {
 		if (!ObjectUtils.isEmpty(context.getCollectionName())) {
 			keyValues = keyValues
 					.and(LowCardinalityCommandKeyNames.MONGODB_COLLECTION.withValue(context.getCollectionName()));
+		} else {
+			keyValues = keyValues.and(LowCardinalityCommandKeyNames.MONGODB_COLLECTION.withValue(KeyValue.NONE_VALUE));
 		}
 
 		if(context.getCommandStartedEvent() == null) {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/observability/MongoObservationCommandListenerTests.java

@@ -20,6 +20,7 @@
 
 import io.micrometer.common.KeyValues;
 import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.Tags;
 import io.micrometer.core.instrument.observation.DefaultMeterObservationHandler;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 import io.micrometer.observation.Observation;
@@ -98,6 +99,20 @@ void commandStartedShouldNotInstrumentWhenNoParentSampleInRequestContext() {
 		assertThat(meterRegistry).hasMeterWithName("spring.data.mongodb.command.active");
 	}
 
+	@Test
+	void commandStartedShouldIncludeCollectionIfMissing() {
+
+		// when
+		listener.commandStarted(new CommandStartedEvent(new MapRequestContext(), 0, 0, null, "some name", "hello", null));
+
+		// then
+		// although command 'hello' is collection-less, metric must have tag "db.mongodb.collection"
+		assertThat(meterRegistry).hasMeterWithNameAndTags(
+				"spring.data.mongodb.command.active",
+				Tags.of("db.mongodb.collection", "none"));
+
+	}
+
 	@Test
 	void successfullyCompletedCommandShouldCreateTimerWhenParentSampleInRequestContext() {