feat(otel): add OpenTelemetry base instrumentation

[dimoschi]

Summary

Adds OpenTelemetry instrumentation to yente (ref #1079):

• FastAPI auto-instrumentation for HTTP metrics and traces via FastAPIInstrumentor
• Manual spans on all SearchProvider operations via __init_subclass__, providing consistent tracing for both Elasticsearch and OpenSearch backends
• Uses stable OTEL semantic conventions (db.system.name, db.operation.name)
• No custom configuration - no manual TracerProvider, no new yente env vars. Users configure everything via standard OTEL_* env vars and the opentelemetry-instrument CLI

New dependencies

• opentelemetry-api
• opentelemetry-sdk
• opentelemetry-instrumentation-fastapi
• opentelemetry-exporter-otlp-proto-http

Usage

To enable OTEL tracing, run yente with opentelemetry-instrument:

opentelemetry-instrument \
  --service_name yente \
  --traces_exporter otlp \
  --exporter_otlp_endpoint http://collector:4318 \
  yente serve

Or configure via env vars:

OTEL_SERVICE_NAME=yente
OTEL_TRACES_EXPORTER=otlp
OTEL_EXPORTER_OTLP_ENDPOINT=http://collector:4318
opentelemetry-instrument yente serve

Without opentelemetry-instrument, OTEL is a no-op (zero overhead).

What this does NOT include (future PRs)

• /metrics Prometheus scrape endpoint
• Custom gauges for catalog data (datasets loaded/outdated, index staleness)

Test plan

• New tests in tests/test_otel.py verify span creation for HTTP requests and SearchProvider operations
• Existing test suite passes without regressions (104 tests)

[leonhandreke]

Hi, thanks for sending this and sorry for the delay! I was out for part of the week and currently working on some urgent things - will get to this beginning of next week.