docs(observability): add actor application telemetry best practices

[Da Huang (git286)]

The observability guide covers telemetry emitted by Substrate's own control plane, but says nothing to actor authors whose application code emits OpenTelemetry metrics and spans. Those signals sit in the OTel SDK's in-memory buffers (PeriodicReader accumulators, BatchSpanProcessor queues) and are subject to actor lifecycle events in ways a normal long-lived process never is.

Add a section documenting the actual behavior:

• onPause: Full preserves the buffer; onPause: Data discards it, since process memory is excluded from the snapshot and the containers are deleted at the end of every checkpoint.
• The workload receives no SIGTERM and no pre-suspend callback, so a flush can only happen where the actor itself initiates the suspend.
• On resume, gVisor's CLOCK_MONOTONIC includes the suspended interval, so overdue export timers fire immediately.
• Metric data points are stamped at collection time and so are attributed to the resume; span timestamps are recorded when the span ran and so go stale across a long suspension.
• The OTLP connection does not survive a pod migration.

Along with the corresponding recommendations: flush before self-suspend, stay on onPause: Full, shorten the metric and span export timers separately, configure gRPC keepalives, and use a DurableDir volume under onPause: Data.

Known gaps are tracked in #450 (PreSuspend/PostResume hooks), #23 (graceful termination) and #503.

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[Eitan Yarmush (EItanya)]

I think these notes will be important, but I think it's worth waiting until we have some specific direction on the self-suspend and graceful-suspend before putting this doc out there as it will be superseded by that