Add distribution summary metric for datafetcher calls

This commit adds a new `"graphql.request.datafetch.count"` distribution
summary metric for counting the number of data fetching calls per
request.

Closes gh-146

graphql-spring-boot-starter/src/main/java/org/springframework/graphql/boot/actuate/metrics/GraphQlMetricsInstrumentation.java

@@ -17,6 +17,7 @@
 package org.springframework.graphql.boot.actuate.metrics;
 
 import java.util.concurrent.CompletionStage;
+import java.util.concurrent.atomic.AtomicLong;
 
 import graphql.ExecutionResult;
 import graphql.execution.instrumentation.InstrumentationContext;
@@ -26,6 +27,7 @@ import graphql.execution.instrumentation.SimpleInstrumentationContext;
 import graphql.execution.instrumentation.parameters.InstrumentationExecutionParameters;
 import graphql.execution.instrumentation.parameters.InstrumentationFieldFetchParameters;
 import graphql.schema.DataFetcher;
+import io.micrometer.core.instrument.DistributionSummary;
 import io.micrometer.core.instrument.MeterRegistry;
 import io.micrometer.core.instrument.Tag;
 import io.micrometer.core.instrument.Timer;
@@ -41,10 +43,16 @@ class GraphQlMetricsInstrumentation extends SimpleInstrumentation {
 
 	private final AutoTimer autoTimer;
 
+	private final DistributionSummary dataFetchingSummary;
+
 	GraphQlMetricsInstrumentation(MeterRegistry registry, GraphQlTagsProvider tagsProvider, AutoTimer autoTimer) {
 		this.registry = registry;
 		this.tagsProvider = tagsProvider;
 		this.autoTimer = autoTimer;
+		this.dataFetchingSummary = DistributionSummary.builder("graphql.request.datafetch.count")
+				.baseUnit("calls")
+				.description("Count of DataFetcher calls per request.")
+				.register(this.registry);
 	}
 
 	@Override
@@ -66,9 +74,10 @@ class GraphQlMetricsInstrumentation extends SimpleInstrumentation {
 					if (!result.getErrors().isEmpty()) {
 						result.getErrors()
 								.forEach((error) -> GraphQlMetricsInstrumentation.this.registry.counter("graphql.error",
-										GraphQlMetricsInstrumentation.this.tagsProvider.getErrorTags(parameters, error))
+												GraphQlMetricsInstrumentation.this.tagsProvider.getErrorTags(parameters, error))
 										.increment());
 					}
+					GraphQlMetricsInstrumentation.this.dataFetchingSummary.record(state.getDataFetchingCount());
 				}
 			};
 		}
@@ -92,7 +101,6 @@ class GraphQlMetricsInstrumentation extends SimpleInstrumentation {
 						recordDataFetcherMetric(sample, dataFetcher, parameters, null);
 						return value;
 					}
-
 				}
 				catch (Throwable throwable) {
 					recordDataFetcherMetric(sample, dataFetcher, parameters, throwable);
@@ -108,6 +116,8 @@ class GraphQlMetricsInstrumentation extends SimpleInstrumentation {
 		Timer.Builder timer = this.autoTimer.builder("graphql.datafetcher");
 		timer.tags(this.tagsProvider.getDataFetchingTags(dataFetcher, parameters, throwable));
 		sample.stop(timer.register(this.registry));
+		RequestMetricsInstrumentationState state = parameters.getInstrumentationState();
+		state.incrementDataFetchingCount();
 	}
 
 	static class RequestMetricsInstrumentationState implements InstrumentationState {
@@ -118,6 +128,8 @@ class GraphQlMetricsInstrumentation extends SimpleInstrumentation {
 
 		private Timer.Sample sample;
 
+		private AtomicLong dataFetchingCount = new AtomicLong(0L);
+
 		RequestMetricsInstrumentationState(AutoTimer autoTimer, MeterRegistry registry) {
 			this.timer = autoTimer.builder("graphql.request");
 			this.registry = registry;
@@ -136,6 +148,14 @@ class GraphQlMetricsInstrumentation extends SimpleInstrumentation {
 			this.sample.stop(this.timer.register(this.registry));
 		}
 
+		void incrementDataFetchingCount() {
+			this.dataFetchingCount.incrementAndGet();
+		}
+
+		long getDataFetchingCount() {
+			return this.dataFetchingCount.get();
+		}
+
 	}
 
 }

spring-graphql-docs/src/docs/asciidoc/boot-starter.adoc

@@ -309,6 +309,20 @@ A `DataFetcher` metric timer is available at `/actuator/metrics/graphql.datafetc
 |"SUCCESS", "ERROR"
 |===
 
+[[boot-graphql-metrics-datafetcher-summary]]
+=== `DataFetcher` Distribution Summary
+
+A https://micrometer.io/docs/concepts#_distribution_summaries[distribution summary]
+that counts the number of non-trivial `DataFetcher` calls made per request.
+This metric is useful for detecting "N+1" data fetching issues and consider batch loading;
+it provides the `"TOTAL"` number of data fetcher calls made over the `"COUNT"` of recorded requests,
+as well as the `"MAX"` calls made for a single request over the considered period.
+
+The distribution is available at `/actuator/metrics/graphql.request.datafetch.count`.
+
+More options are available for
+{spring-boot-ref-docs}/application-properties.html#application-properties.actuator.management.metrics.distribution.maximum-expected-value[configuring distributions with application properties].
+
 
 [[boot-graphql-metrics-error-counter]]
 === Error Counter