Add graphql query complexity histogram metric (#2349)

## Linked Issues/PRs
Closes #2274

## Checklist
- [x] Breaking changes are clearly marked as such in the PR description
and changelog
- [x] New behavior is reflected in tests
- [x] [The specification](https://github.com/FuelLabs/fuel-specs/)
matches the implemented behavior (link update PR if changes are needed)

### Before requesting review
- [x] I have reviewed the code myself
- [x] I have created follow-up issues caused by this PR and linked them
here

Co-authored-by: Green Baneling <[email protected]>

CHANGELOG.md

@@ -18,6 +18,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 - [2335](https://github.com/FuelLabs/fuel-core/pull/2335): Added CLI arguments for configuring GraphQL query costs.
 
+### Added
+- [2347](https://github.com/FuelLabs/fuel-core/pull/2347): Add GraphQL complexity histogram
+
 ## [Version 0.39.0]
 
 ### Added

crates/fuel-core/src/graphql_api/metrics_extension.rs

@@ -6,11 +6,14 @@ use async_graphql::{
         NextParseQuery,
         NextRequest,
         NextResolve,
+        NextValidation,
         ResolveInfo,
     },
     parser::types::ExecutableDocument,
     Response,
+    ServerError,
     ServerResult,
+    ValidationResult,
     Value,
     Variables,
 };
@@ -118,4 +121,14 @@ impl Extension for MetricsExtInner {
 
         res
     }
+
+    async fn validation(
+        &self,
+        ctx: &ExtensionContext<'_>,
+        next: NextValidation<'_>,
+    ) -> Result<ValidationResult, Vec<ServerError>> {
+        let result = next.run(ctx).await?;
+        graphql_metrics().graphql_complexity_observe(result.complexity as f64);
+        Ok(result)
+    }
 }

crates/metrics/src/graphql_metrics.rs

@@ -22,16 +22,23 @@ pub struct GraphqlMetrics {
     // using gauges in case blocks are rolled back for any reason
     pub total_txs_count: Gauge,
     requests: Family<Label, Histogram>,
+    queries_complexity: Histogram,
 }
 
 impl GraphqlMetrics {
     fn new() -> Self {
         let tx_count_gauge = Gauge::default();
+        let queries_complexity = Histogram::new(buckets_complexity());
         let requests = Family::<Label, Histogram>::new_with_constructor(|| {
             Histogram::new(timing_buckets().iter().cloned())
         });
         let mut registry = global_registry().registry.lock();
         registry.register("graphql_request_duration_seconds", "", requests.clone());
+        registry.register(
+            "graphql_query_complexity",
+            "The complexity of all queries received",
+            queries_complexity.clone(),
+        );
 
         registry.register(
             "importer_tx_count",
@@ -41,6 +48,7 @@ impl GraphqlMetrics {
 
         Self {
             total_txs_count: tx_count_gauge,
+            queries_complexity,
             requests,
         }
     }
@@ -51,9 +59,30 @@ impl GraphqlMetrics {
         });
         histogram.observe(time);
     }
+
+    pub fn graphql_complexity_observe(&self, complexity: f64) {
+        self.queries_complexity.observe(complexity);
+    }
 }
 
 static GRAPHQL_METRICS: OnceLock<GraphqlMetrics> = OnceLock::new();
 pub fn graphql_metrics() -> &'static GraphqlMetrics {
     GRAPHQL_METRICS.get_or_init(GraphqlMetrics::new)
 }
+
+fn buckets_complexity() -> impl Iterator<Item = f64> {
+    [
+        1_000.0,
+        5_000.0,
+        10_000.0,
+        20_000.0,
+        50_000.0,
+        100_000.0,
+        250_000.0,
+        500_000.0,
+        1_000_000.0,
+        5_000_000.0,
+        10_000_000.0,
+    ]
+    .into_iter()
+}