feat(diagnostics-otel): add exporter health diagnostics

Adds diagnostics-otel exporter health events and signal-specific endpoint wiring, with docs and config schema coverage.

docs/gateway/configuration-reference.md

@@ -869,6 +869,9 @@ Notes:
     otel: {
       enabled: false,
       endpoint: "https://otel-collector.example.com:4318",
+      tracesEndpoint: "https://traces.example.com/v1/traces",
+      metricsEndpoint: "https://metrics.example.com/v1/metrics",
+      logsEndpoint: "https://logs.example.com/v1/logs",
       protocol: "http/protobuf", // http/protobuf | grpc
       headers: { "x-tenant-id": "my-org" },
       serviceName: "openclaw-gateway",
@@ -903,6 +906,7 @@ Notes:
 - `stuckSessionWarnMs`: age threshold in ms for emitting stuck-session warnings while a session remains in processing state.
 - `otel.enabled`: enables the OpenTelemetry export pipeline (default: `false`). For the full configuration, signal catalog, and privacy model, see [OpenTelemetry export](/gateway/opentelemetry).
 - `otel.endpoint`: collector URL for OTel export.
+- `otel.tracesEndpoint` / `otel.metricsEndpoint` / `otel.logsEndpoint`: optional signal-specific OTLP endpoints. When set, they override `otel.endpoint` for that signal only.
 - `otel.protocol`: `"http/protobuf"` (default) or `"grpc"`.
 - `otel.headers`: extra HTTP/gRPC metadata headers sent with OTel export requests.
 - `otel.serviceName`: service name for resource attributes.
@@ -912,6 +916,7 @@ Notes:
 - `otel.captureContent`: opt-in raw content capture for OTEL span attributes. Defaults to off. Boolean `true` captures non-system message/tool content; the object form lets you enable `inputMessages`, `outputMessages`, `toolInputs`, `toolOutputs`, and `systemPrompt` explicitly.
 - `OTEL_SEMCONV_STABILITY_OPT_IN=gen_ai_latest_experimental`: environment toggle for latest experimental GenAI span provider attributes. By default spans keep the legacy `gen_ai.system` attribute for compatibility; GenAI metrics use bounded semantic attributes.
 - `OPENCLAW_OTEL_PRELOADED=1`: environment toggle for hosts that already registered a global OpenTelemetry SDK. OpenClaw then skips plugin-owned SDK startup/shutdown while keeping diagnostic listeners active.
+- `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT`, `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT`, and `OTEL_EXPORTER_OTLP_LOGS_ENDPOINT`: signal-specific endpoint env vars used when the matching config key is unset.
 - `cacheTrace.enabled`: log cache trace snapshots for embedded runs (default: `false`).
 - `cacheTrace.filePath`: output path for cache trace JSONL (default: `$OPENCLAW_STATE_DIR/logs/cache-trace.jsonl`).
 - `cacheTrace.includeMessages` / `includePrompt` / `includeSystem`: control what is included in cache trace output (all default: `true`).