demos: add actor telemetry continuity reference demo (#503)

[sanskar singh bhardwaj (sanskar-singh-2403)]

Add a reference demo for actor telemetry continuity across suspend/resume, the
follow-up work requested in #503.

The demo is a small Go HTTP server that emits OpenTelemetry metrics and trace
spans over OTLP on every request. It reproduces the #503 loss: an OTel push
exporter batches telemetry in memory and flushes on a timer, so when the push
interval (default 60s) is longer than the active window before idle suspension,
telemetry is still buffered in the SDK when the actor is suspended. With
snapshotsConfig.onPause: Data (the mode the template uses) Substrate drops
process RAM on pause, so that buffered telemetry is lost.

The mitigation is isolated behind a flushTelemetry(ctx) seam that ForceFlushes
both the MeterProvider and TracerProvider. Today it is called from the
self-suspend path via an idle watcher; when the PreSuspend hook (#450) lands,
flushTelemetry does not change, only its call-site moves into the PreSuspend
handler. This keeps #450 a call-site move rather than a rewrite.

The ActorTemplate sets OTEL_EXPORTER_OTLP_ENDPOINT to the in-cluster
collector, since the SDK default (localhost:4317) has nothing listening inside
the actor sandbox netns.

Includes install-demo-actor-telemetry.sh wired into install-ate.sh, a
rendered-and-validated template, unit tests for the idle-flush watcher, and a
README documenting the reproduce/verify flow against Jaeger and Prometheus on
Kind.

Scope

This covers the "reference demo for actor telemetry continuity" action item in
#503 only. The docs/observability.md update is tracked separately, and the
PreSuspend/PostResume alignment is #450.

Part of #503

Testing

• gofmt, go vet, go build, and go test ./demos/actor-telemetry/ all pass
  (4/4 unit tests on the idle-flush watcher).

• Template renders via the install script's sed substitution and parses as
  valid Kubernetes manifests.

• End-to-end suspend/resume verification against a live Kind cluster with
  ate-system is described in the README.

• Tests pass

• Appropriate changes to documentation are included in the PR