Create the foundational metrics module using OpenTelemetry Metrics API with support for counters, histograms, and gauges

[psschwei]

Description:
Create the foundational metrics module using OpenTelemetry Metrics API with support for counters, histograms, and gauges.

Detailed Requirements:

1. Create mellea/telemetry/metrics.py module with:
   • Lazy initialization of MeterProvider (similar to tracing pattern)
   • Environment-based configuration (MELLEA_METRICS_ENABLED)
   • Zero overhead when disabled (no-op instruments)
   • Named meter: mellea.metrics
2. Define base instrument creation helpers:
   • create_counter(name, description, unit)
   • create_histogram(name, description, unit, boundaries)
   • create_up_down_counter(name, description, unit)
3. Add dependencies to optional [telemetry] extra:
   • opentelemetry-sdk (already present)
   • Ensure metrics API available

Files to Create:

• mellea/telemetry/metrics.py

Files to Modify:

• mellea/telemetry/__init__.py - Export metrics functions
• pyproject.toml - Verify metrics dependencies in telemetry extra

Environment Variables:

Variable	Default	Description
MELLEA_METRICS_ENABLED	false	Enable metrics collection
MELLEA_METRICS_CONSOLE	false	Print metrics to console (debug)

Acceptance Criteria:

• MeterProvider initialized on first metric creation
• Instruments are no-ops when metrics disabled
• Counter, histogram, up-down-counter creation works
• No import errors when opentelemetry not installed

[ajbozarth]

I'm going to look into this and try picking this up if no one has already