fix(telemetry): make MetricsClient singleton thread-safe

[gtholpadi]

Motivation

The MetricsClient singleton uses a pattern where __new__ creates the instance and __init__ sets up OpenTelemetry instruments. Without synchronization, concurrent threads can observe a partially-initialized instance — one thread creates the object while another thread gets that same object before create_instruments() has assigned all the counter/histogram attributes. This causes intermittent AttributeError crashes when running multiple agents in parallel via ThreadPoolExecutor.

Resolves: #2342

Public API Changes

No public API changes. The fix is internal to MetricsClient's initialization logic. Existing single-threaded and multi-threaded usage continues to work identically — the singleton is simply safe to access concurrently now.

Use Cases

• Concurrent agent execution: Running multiple Agent instances in a ThreadPoolExecutor without needing to manually pre-initialize MetricsClient() on the main thread first.

Description

Add threading.Lock with double-checked locking to MetricsClient's new and init methods to prevent race conditions when multiple threads concurrently initialize the singleton. Without this fix, concurrent agent invocations can intermittently raise AttributeError when accessing instrument attributes before create_instruments() completes.

Type of Change

Bug fix

Testing

Tested locally via new unit test and existing integration tests.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly --> NO CHANGE
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
strands-py/src/strands/telemetry/metrics.py	91.66%	0 Missing and 1 partial ⚠️

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