fix(metrics): return errors instead of panic/log.Fatalf in library code

[k4leung4]

Summary

• SetupTracer now returns (func(), error) instead of panicking on failure
• RegisterAndServe/RegisterListenAndServe use clog instead of stdlib log, and ignore http.ErrServerClosed on shutdown
• Fix log.Fatalf → t.Fatalf in tls_test.go
• Remove stdlib log import from metrics/server.go

Motivation

Library code should not call panic or log.Fatalf — these terminate the entire process and prevent callers from implementing graceful error handling. ~149 callers in mono use defer metrics.SetupTracer(ctx)() which would crash if the trace exporter fails to initialize.

Breaking change

SetupTracer signature: func(context.Context) func() → func(context.Context) (func(), error)

Callers need to change from:

defer metrics.SetupTracer(ctx)()

to:

shutdownTracer, err := metrics.SetupTracer(ctx)
if err != nil {
    log.Fatalf("SetupTracer() = %v", err)
}
defer shutdownTracer()

Test plan

• go test ./... -count=1 -race passes
• CI passes

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