Add subscription/defer observability attributes and metrics

.changesets/feat_rohan_b99_subscription_observability_heartbeat_payload_errors.md

@@ -0,0 +1,10 @@
+### Add `apollo.subscription.end_reason` and `apollo.defer.end_reason` attributes to router spans ([PR #8858](https://github.com/apollographql/router/pull/8858))
+
+Adds two new span attributes that indicate why a streaming response (subscription or defer) ended:
+
+- **`apollo.subscription.end_reason`**: Records the reason a subscription was terminated. Possible values are `server_close`, `stream_end`, `heartbeat_delivery_failed`, `client_disconnect`, `schema_reload`, and `config_reload`.
+- **`apollo.defer.end_reason`**: Records the reason a deferred query ended. Possible values are `completed` (all deferred chunks were delivered successfully) and `client_disconnect` (the client disconnected before all deferred data was delivered).
+
+Both attributes are added dynamically to router spans only when relevant (i.e., only on requests that actually use subscriptions or `@defer`), rather than being present on every router span.
+
+By [@rohan-b99](https://github.com/rohan-b99) in https://github.com/apollographql/router/pull/8858

apollo-router/src/plugins/subscription/execution.rs

@@ -35,8 +35,8 @@ use crate::services::execution;
 use crate::services::execution::QueryPlan;
 use crate::services::subgraph::BoxGqlStream;
 
-const SUBSCRIPTION_CONFIG_RELOAD_EXTENSION_CODE: &str = "SUBSCRIPTION_CONFIG_RELOAD";
-const SUBSCRIPTION_SCHEMA_RELOAD_EXTENSION_CODE: &str = "SUBSCRIPTION_SCHEMA_RELOAD";
+pub(crate) const SUBSCRIPTION_CONFIG_RELOAD_EXTENSION_CODE: &str = "SUBSCRIPTION_CONFIG_RELOAD";
+pub(crate) const SUBSCRIPTION_SCHEMA_RELOAD_EXTENSION_CODE: &str = "SUBSCRIPTION_SCHEMA_RELOAD";
 const SUBSCRIPTION_JWT_EXPIRED_EXTENSION_CODE: &str = "SUBSCRIPTION_JWT_EXPIRED";
 const SUBSCRIPTION_EXECUTION_ERROR_EXTENSION_CODE: &str = "SUBSCRIPTION_EXECUTION_ERROR";
 

apollo-router/src/plugins/subscription/mod.rs

@@ -36,6 +36,8 @@ pub(crate) mod notification;
 pub(crate) mod subgraph;
 
 pub(crate) use callback::SUBSCRIPTION_CALLBACK_HMAC_KEY;
+pub(crate) use execution::SUBSCRIPTION_CONFIG_RELOAD_EXTENSION_CODE;
+pub(crate) use execution::SUBSCRIPTION_SCHEMA_RELOAD_EXTENSION_CODE;
 pub(crate) use execution::SubscriptionExecutionLayer;
 pub(crate) use execution::SubscriptionTaskParams;
 pub(crate) use fetch::fetch_service_handle_subscription;

apollo-router/src/plugins/telemetry/span_factory.rs

@@ -93,7 +93,7 @@ impl SpanMode {
                     "otel.status_code" = ::tracing::field::Empty,
                     "apollo_private.duration_ns" = ::tracing::field::Empty,
                     "apollo_private.http.request_headers" = ::tracing::field::Empty,
-                    "apollo_private.http.response_headers" = ::tracing::field::Empty
+                    "apollo_private.http.response_headers" = ::tracing::field::Empty,
                 );
                 span
             }
@@ -163,7 +163,7 @@ impl SpanMode {
                     apollo_private.graphql.variables = Telemetry::filter_variables_values(
                         &request.supergraph_request.body().variables,
                         &send_variable_values,
-                    )
+                    ),
                 )
             }
         }