SDK instrumentation: OpenTelemetry opt-in for Python + Go docs

[zeevdr]

Add OpenTelemetry instrumentation support to SDKs so users can see get/set/watch operations in their application traces.

Python SDK (medium effort):

• Add otel optional extra (pip install opendecree[otel])
• otel=True constructor flag wires opentelemetry-instrumentation-grpc interceptor
• Works for sync + async clients and watchers

Go SDKs (docs only):

• Users already pass their own grpc.ClientConn — document the OTel interceptor pattern
• No code changes needed

Effort: .claude/efforts/22-sdk-instrumentation.md

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Migrated from zeevdr/decree#72

[zeevdr]

All work complete:

• Python SDK: feat(sdk): add otel=True instrumentation flag for Python SDK decree-python#117 — otel=True flag, lazy imports, pip install opendecree[otel], 100% coverage
• Go SDKs: docs(sdk): add Go OTel instrumentation guide #630 — OTel interceptor pattern docs

No code changes needed for Go (users own the grpc.ClientConn).