Phase 1c: RPC layer telemetry integration

Add tracing instrumentation to the RPC request handling layer:

TracingInstrumentation.h:
- Convenience macros (XRPL_TRACE_RPC, XRPL_TRACE_TX, etc.) that
  create RAII SpanGuard objects when telemetry is enabled
- XRPL_TRACE_SET_ATTR / XRPL_TRACE_EXCEPTION for span enrichment
- Zero-overhead no-ops when XRPL_ENABLE_TELEMETRY is not defined

RPCHandler.cpp:
- Trace each RPC command with span name "rpc.command.<method>"
- Record command name, API version, role, and status as attributes
- Capture exceptions on the span for error visibility

ServerHandler.cpp:
- Trace HTTP requests ("rpc.request"), WebSocket messages
  ("rpc.ws_message"), and processRequest ("rpc.process")

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/scripts/levelization/results/ordering.txt

@@ -259,6 +259,7 @@ xrpld.perflog > xrpl.json
 xrpld.rpc > xrpl.basics
 xrpld.rpc > xrpl.core
 xrpld.rpc > xrpld.core
+xrpld.rpc > xrpld.telemetry
 xrpld.rpc > xrpl.json
 xrpld.rpc > xrpl.ledger
 xrpld.rpc > xrpl.net
@@ -269,3 +270,4 @@ xrpld.rpc > xrpl.resource
 xrpld.rpc > xrpl.server
 xrpld.rpc > xrpl.tx
 xrpld.shamap > xrpl.shamap
+xrpld.telemetry > xrpl.telemetry