Add concurrency tests for meter/instrument methods

[chimchim89]

Description

Addresses FIXME comments in the metrics API test files that indicated concurrency tests were needed to verify that meter and instrument methods can be called concurrently safely. No production code was modified.

Added a TestConcurrency class using Python's threading module to each of the three affected test files:

• test_instruments.py: concurrent tests for Counter.add, UpDownCounter.add, Histogram.record, and Gauge.set
• test_meter.py: concurrent tests for Meter.create_counter, Meter.create_up_down_counter, Meter.create_observable_counter, Meter.create_histogram, Meter.create_gauge, Meter.create_observable_gauge, Meter.create_observable_up_down_counter
• test_meter_provider.py: concurrent tests for NoOpMeterProvider.get_meter and _ProxyMeterProvider.get_meter

Type of change

This is a test-only change that adds missing concurrency tests to address existing FIXME comments. No production code was modified.

How Has This Been Tested?

pytest opentelemetry-api/tests/metrics
76 passed in 0.62s

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[github-actions]

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