Add instrumentation for various basic metrics

[shangyian]

Summary

HTTP / Request Layer

In: datajunction_server/instrumentation/middleware.py

Metric	Type	Tags	Description
dj.request	timer (ms)	route, method, status_code	End-to-end latency for every HTTP request
dj.request.in_flight	gauge	—	Concurrent requests in progress (per instance)
dj.db.pool.size	gauge	—	Configured max pool size
dj.db.pool.checked_out	gauge	—	Active DB connections
dj.db.pool.available	gauge	—	Idle connections (size - checked_out)
dj.db.pool.overflow	gauge	—	Connections above pool size limit

Database Layer

Source: datajunction_server/utils.py

Metric	Type	Tags	Description
dj.db.query_count	timer (count)	session_label	Number of DB queries issued per session/request

Query Cache

In: datajunction_server/internal/caching/query_cache_manager.py

Metric	Type	Tags	Description
dj.cache.hit	counter	query_type	Cache hit on SQL build cache
dj.cache.miss	counter	query_type	Cache miss — fresh SQL build triggered
dj.cache.refresh.pending	gauge	—	Background refresh jobs currently queued
dj.sql.build_latency_ms	timer (ms)	query_type	Time to build SQL from scratch (cache miss path)
dj.cache.key_build_ms	timer (ms)	query_type	Time to compute the versioned cache key
dj.cache.refresh_latency_ms	timer (ms)	query_type	Time for a background refresh-ahead rebuild

DAG Traversal

In: datajunction_server/sql/dag.py

Metric	Type	Tags	Description
dj.dag.traversal_ms	timer (ms)	operation	Latency of DAG traversal functions

operation values: get_downstream_nodes, get_upstream_nodes, get_dimensions_dag

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

[netlify]

✅ Deploy Preview for thriving-cassata-78ae72 canceled.

Name	Link
🔨 Latest commit	7bf1bea
🔍 Latest deploy log	https://app.netlify.com/projects/thriving-cassata-78ae72/deploys/69ab6aa63281c000081c02dd