feat: add impact metric counter

[sjaanus]

Following pattern we did in java.

Does same implementation we had here Unleash/unleash-java-sdk#336

[coveralls]

Pull Request Test Coverage Report for Build 20592912994

Details

• 65 of 65 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 93.984%

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Totals
Change from base Build 19699333839:	0.6%
Covered Lines:	703
Relevant Lines:	748

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💛 - Coveralls

[Copilot]

Pull request overview

This pull request adds an impact metric counter implementation to the Python SDK, following the same pattern established in the Java SDK (unleash-java-sdk PR #336). The implementation provides a thread-safe counter with label support for tracking metrics.

Key changes:

• Implements a CounterImpl class with increment and collect operations
• Adds support for labeled metrics with label serialization/deserialization
• Includes comprehensive unit tests for counter functionality

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 10 comments.

File	Description
UnleashClient/impact_metrics/metric_types.py	Implements the core metric types including MetricType enum, MetricOptions dataclass, Counter protocol, and CounterImpl with thread-safe operations
UnleashClient/impact_metrics/__init__.py	Exports public API for the impact_metrics module
tests/unit_tests/impact_metrics/test_metric_types.py	Provides unit tests for counter increment, labeling, and collect behavior
tests/unit_tests/impact_metrics/__init__.py	Empty init file for test module

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