feat: add prometheus endpoint

[pauliyobo]

Description

This PR allows pgstream to expose a configurable Prometheus endpoint, served from the existing health server, so metrics can be scraped directly via the OTel Prometheus exporter without requiring an OTLP collector.

Related Issue(s)

closes #903

Type of Change

Please select the relevant option(s):

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test coverage improvement
• 🔨 Build/CI changes
• 🧹 Code cleanup

Changes Made

• Add instrumentation.metrics.prometheus config (YAML and env vars) to enable/configure a Prometheus scrape endpoint.
• Serve the endpoint from the health server (default path /metrics, returns 404 when disabled).
• Wire the OTel Prometheus exporter into the meter provider independently of the OTLP exporter, so Prometheus works standalone without an OTLP endpoint configured.
• Update configuration and observability docs for the new option.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Checklist

• Code follows project style guidelines
• Self-review completed
• Code is well-commented
• Documentation updated where necessary

Additional Notes

I have made the endpoint configurable as well. Is that fine, or should I leave /metrics?