compute_ctl: Add OTEL tracing to incoming HTTP requests and startup (#10971)

We lost this with the switch to axum for the HTTP server. Add it back.

In addition to just resurrecting the functionality we had before, pass
the tracing context of the /configure HTTP request to the start_postgres
operation that runs in the main thread. This way, the 'start_postgres'
and all its sub-spans like getting the basebackup become children of the
HTTP request span. This allows end-to-end tracing of a compute start,
all the way from the proxy to the SQL queries executed by compute_ctl as
part of compute startup.

compute_tools/src/http/routes/configure.rs

@@ -45,13 +45,18 @@ pub(in crate::http) async fn configure(
             return JsonResponse::invalid_status(state.status);
         }
 
+        // Pass the tracing span to the main thread that performs the startup,
+        // so that the start_compute operation is considered a child of this
+        // configure request for tracing purposes.
+        state.startup_span = Some(tracing::Span::current());
+
         state.pspec = Some(pspec);
         state.set_status(ComputeStatus::ConfigurationPending, &compute.state_changed);
         drop(state);
     }
 
     // Spawn a blocking thread to wait for compute to become Running. This is
-    // needed to do not block the main pool of workers and be able to serve
+    // needed to not block the main pool of workers and to be able to serve
     // other requests while some particular request is waiting for compute to
     // finish configuration.
     let c = compute.clone();

compute_tools/src/http/server.rs

@@ -121,6 +121,7 @@ impl From<Server> for Router<Arc<ComputeNode>> {
                 )
                 .layer(PropagateRequestIdLayer::x_request_id()),
         )
+            .layer(tower_otel::trace::HttpLayer::server(tracing::Level::INFO))
     }
 }