fix: ADDON-85914 findings fixes

[wtobis-splunk]

Summary

• Fix DELTA temporality on OTLPMetricExporter — both Counter and
  Histogram now explicitly declare AggregationTemporality.DELTA, matching
  the existing LoggerMetricExporter behaviour and ensuring each export
  interval reports only the delta, not a cumulative total.

• Add ObservabilityService.flush() — public method that calls
  MeterProvider.force_flush() via the OTel SDK's public API. Replaces the
  previous pattern of reaching into private SDK internals to drain metric
  readers before process exit.

• Safe lazy import of grpcio — import grpc and OTLPMetricExporter
  are moved from module top-level into _create_otlp_exporter().
  from solnlib.observability import StanzaObservabilityRecorder now succeeds
  in environments where grpcio is not installed.

• Add StanzaObservabilityRecorder — stanza-scoped recorder that wraps
  ObservabilityService with a per-process singleton cache (one service per
  modinput_type). Key design decisions:

  • Singleton held in ClassVar on the class itself, making it
    straightforward to reset in tests.
  • register_instrument(callback) delegates to
    ObservabilityService.register_instrument() — no need to access the
    underlying service directly.
  • ATTR_MODINPUT_NAME is always applied last in record() and cannot be
    overridden by extra_attrs, preserving the stanza-scoped guarantee.
  • Implements context manager protocol; __exit__ calls flush()
    automatically.
  • _emit_zero_baseline() records add(0, 0) on construction so the metric
    series is visible from the first collection cycle even when no events were
    ingested.