Report a metric to Apollo if fragment reuse is enabled (#6267)

Co-authored-by: Bryn Cooke <[email protected]>

apollo-router/src/configuration/metrics.rs

@@ -49,6 +49,8 @@ impl Metrics {
         data.populate_user_plugins_instrument(configuration);
         data.populate_query_planner_experimental_parallelism(configuration);
         data.populate_deno_or_rust_mode_instruments(configuration);
+        data.populate_legacy_fragment_usage(configuration);
+
         data.into()
     }
 }
@@ -494,6 +496,18 @@ impl InstrumentData {
         );
     }
 
+    pub(crate) fn populate_legacy_fragment_usage(&mut self, configuration: &Configuration) {
+        // Fragment generation takes precedence over fragment reuse. Only report when fragment reuse is *actually active*.
+        if configuration.supergraph.reuse_query_fragments == Some(true)
+            && !configuration.supergraph.generate_query_fragments
+        {
+            self.data.insert(
+                "apollo.router.config.reuse_query_fragments".to_string(),
+                (1, HashMap::new()),
+            );
+        }
+    }
+
     pub(crate) fn populate_query_planner_experimental_parallelism(
         &mut self,
         configuration: &Configuration,