fix(telemetry): phase-6 dashboards — rename UIDs, add $node filter, drop line numbers

Phase-6 introduces ledger-operations, peer-network, and the five StatsD
dashboards. Align them with the rest of the chain:

- Rename dashboard UIDs from `rippled-*` to `xrpld-*` so the provisioned
  UIDs match the post-rename-script documentation (`docs.sh` rewrites
  .md but not .json, so the two drifted). Runbook references
  `xrpld-rpc-perf`, `xrpld-transactions`, etc., now the JSON matches.
- Add the `$node` template variable + `exported_instance=~"$node"` filter
  to every target in the five `statsd-*` dashboards. Mirrors the pattern
  already used by consensus-health, ledger-operations, and peer-network
  per the project rule that every dashboard must support per-node
  filtering.
- Strip `:<line>` (and `:NN-NN` range) suffixes from C++ file references
  in every dashboard panel description and in docker/telemetry/TESTING.md.
  Line numbers drift on every refactor; the filename alone is enough to
  grep.
- Replace stale `rpc.request` entries with the real emitted span names
  (`rpc.http_request`, `rpc.ws_upgrade`, `rpc.ws_message`, `rpc.process`)
  in TESTING.md so operators can copy-paste the filters and hit real
  traces.
- Also drop the `:706` line ref from the `StatsDCollector.cpp` callout
  in `06-implementation-phases.md`.

OpenTelemetryPlan/06-implementation-phases.md

@@ -350,7 +350,7 @@ xrpld has a mature metrics framework (`beast::insight`) that emits StatsD-format
 
 ### Wire Format Fix (Task 6.1) — DEFERRED
 
-The `StatsDMeterImpl` in `StatsDCollector.cpp:706` sends metrics with `|m` suffix, which is non-standard StatsD. The OTel StatsD receiver silently drops these. Fix: change `|m` to `|c` (counter), which is semantically correct since meters are increment-only counters. Only 2 metrics are affected (`warn`, `drop` in Resource Manager).
+The `StatsDMeterImpl` in `StatsDCollector.cpp` sends metrics with `|m` suffix, which is non-standard StatsD. The OTel StatsD receiver silently drops these. Fix: change `|m` to `|c` (counter), which is semantically correct since meters are increment-only counters. Only 2 metrics are affected (`warn`, `drop` in Resource Manager).
 
 **Status**: Deferred as a separate change — this is a breaking change for any StatsD backend that previously consumed the custom `|m` type. The Resource Warnings and Resource Drops dashboard panels will show no data until this fix is applied.