feat(telemetry): add Phase 5 documentation, deployment configs, and integration tests

Add the observability stack deployment infrastructure and integration
test framework for verifying end-to-end trace export.

- Add Grafana dashboards: RPC performance, transaction overview,
  consensus health (pre-provisioned via dashboards.yaml)
- Add Prometheus config for spanmetrics collection from OTel Collector
- Update OTel Collector config with spanmetrics connector and
  prometheus exporter for RED metrics
- Add docker-compose services: prometheus, dashboard provisioning
- Add integration-test.sh with Tempo API-based span verification
  (replaces previous Jaeger-based approach)
- Add TESTING.md with step-by-step deployment and verification guide
- Add telemetry-runbook.md for production operations reference
- Add xrpld-telemetry.cfg sample configuration
- Add toDisplayString() for ConsensusMode (human-readable span values)
- Update Phase 2/3 task lists with known issues sections
- Add Phase 5 integration test task list
- Add TraceContext protobuf fields for future relay propagation
- Wire telemetry lifecycle (setServiceInstanceId/start/stop) in
  Application.cpp

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/xrpl/proto/xrpl.proto

@@ -91,6 +91,10 @@ message TraceContext {
   optional bytes trace_id = 1;      // 16-byte trace identifier
   optional bytes span_id = 2;       // 8-byte parent span identifier
   optional uint32 trace_flags = 3;  // bit 0 = sampled
+  // TODO: trace_state is reserved for W3C tracestate vendor-specific
+  // key-value pairs but is not yet read or written by
+  // TraceContextPropagator. Wire it when cross-vendor trace
+  // propagation is needed.
   optional string trace_state = 4;  // W3C tracestate header value
 }
 

include/xrpl/telemetry/TraceContextPropagator.h

@@ -6,6 +6,13 @@
     Protocol Buffer TraceContext messages (P2P cross-node propagation).
 
     Only compiled when XRPL_ENABLE_TELEMETRY is defined.
+
+    TODO: These utilities are not yet wired into the P2P message flow.
+    To enable cross-node distributed traces, call injectToProtobuf() in
+    PeerImp when sending TMTransaction/TMProposeSet messages, and call
+    extractFromProtobuf() in the corresponding message handlers to
+    reconstruct the parent span context before starting a child span.
+    This was deferred to validate single-node tracing performance first.
 */
 
 #ifdef XRPL_ENABLE_TELEMETRY