feat(relay): OpenTelemetry baseline — tracing exporter + Axum instrumentation (MM-137) · pull #17

Summary#

Adds a [telemetry] config section (opt-in, off by default) with OTLP gRPC export via opentelemetry-otlp + tonic
New crates/relay/src/telemetry.rs: layered subscriber — fmt layer always present, OTel layer added when telemetry.enabled = true; OtelGuard flushes buffered spans on graceful shutdown via Drop
OtelMakeSpan in app.rs replaces the default TraceLayer::make_span_with; extracts W3C traceparent/tracestate headers via TraceContextPropagator so upstream proxy traces are joined
#[tracing::instrument] on open_pool and run_migrations with db.system = "sqlite" span attribute
Env overrides: EZPDS_TELEMETRY_ENABLED, EZPDS_OTLP_ENDPOINT, OTEL_SERVICE_NAME
7 new config tests; 72 tests pass; clippy clean

cargo test --workspace passes
cargo clippy --workspace -- -D warnings clean
Default config (telemetry.enabled = false) — stdout only, no OTLP connection
With telemetry.enabled = true + docker run -d --name jaeger -p 16686:16686 -p 4317:4317 jaegertracing/all-in-one:latest — traces appear in Jaeger at http://localhost:16686
Send request with traceparent header — Jaeger shows it as child of upstream trace