Add clear() method to CollectorRegistry

[veeceey]

Summary

Adds a clear() method to CollectorRegistry that removes all collectors from the registry at once. This reduces boilerplate when clearing registries, as users no longer need to manually unregister each collector individually.

Changes

• Added clear() method to CollectorRegistry class in prometheus_client/registry.py
• Added test case test_clear_works() to verify the method works correctly
• The method clears all collectors and resets the target_info

Usage Example

Before:

registry = CollectorRegistry()
g1 = Gauge("Metric1", "Description1", registry=registry)
g2 = Gauge("Metric2", "Description2", registry=registry)
g3 = Gauge("Metric3", "Description3", registry=registry)

# Remove the gauges from the registry manually
registry.unregister(g1)
registry.unregister(g2)
registry.unregister(g3)

After:

registry = CollectorRegistry()
g1 = Gauge("Metric1", "Description1", registry=registry)
g2 = Gauge("Metric2", "Description2", registry=registry)
g3 = Gauge("Metric3", "Description3", registry=registry)

# Remove all collectors at once
registry.clear()

Test Plan

• Added comprehensive test case that verifies:
  • Multiple metrics can be registered
  • clear() removes all collectors
  • After clearing, the same metric names can be registered again without conflicts
• All existing tests still pass

Fixes #1141