Prometheus metrics endpoint

[dlorenc]

Overview

Add a GET /metrics endpoint in Prometheus format so the service can be scraped by Prometheus or Cloud Monitoring. Split from #104.

Metrics to expose

Metric	Type	Labels
docstore_http_requests_total	Counter	method, path_template, status_code
docstore_http_request_duration_seconds	Histogram	method, path_template
docstore_db_query_duration_seconds	Histogram	query_name
docstore_commits_total	Counter	repo
docstore_merges_total	Counter	repo, result (ok/conflict/policy_denied)
docstore_active_branches	Gauge	repo
docstore_documents_total	Gauge	repo

Implementation

• Add prometheus/client_golang dependency
• Register metrics in a central metrics.go file
• Wire HTTP middleware to record request counts and durations with normalized path_template labels (avoid high-cardinality repo/branch names in labels)
• Expose GET /metrics on the main mux (or a separate internal port)
• Gauge metrics (active_branches, documents_total) updated on write operations or via periodic background collection

Configuration

DOCSTORE_METRICS_ENABLED=true   # default true
DOCSTORE_METRICS_PORT=9090      # separate port recommended to avoid exposing to IAP

Notes

• path_template label should use patterns like /repos/{name}/-/commit not actual repo names, to keep cardinality bounded
• Metrics port should not be behind IAP — it's for internal scraping only