internal/tracer: only start OTEL spans if ShouldTrace is true

[bobheadxi]

Currently we rely on implementors to explicitly check ShouldTrace. This works implicitly through internal/trace/ot which guards against ShouldTrace and returns no-op OpenTracing tracers, which worked until we started moving code over to use OpenTelemetry libraries directly, where a similar guard was not implemented.

This wraps all tracers provided from the global switchable oteltrace.TracerProvider to only start spans when ShouldTrace evaluates to true without the caller having to do anything explicitly. Also updates ShouldTrace to respect global trace-all without requiring the context set.

Test plan

sg start and see all the no-op logs coming through with observability.tracing: { debug: true } with sampling: selective or sampling: none

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 9eef7919deb7edef6d0689bdddc2d91dc8ad2b06...c65902d4e9b1cc2965520d785f7c18ba7e990283.

Notify	File(s)
@keegancsmith	internal/trace/policy/policy.go internal/tracer/should_trace.go internal/tracer/switchable_otel.go internal/tracer/watch_test.go
@sourcegraph/dev-experience	internal/trace/policy/policy.go internal/tracer/should_trace.go internal/tracer/switchable_otel.go internal/tracer/watch_test.go