Exemplar filter — trace_based only.

Spec metrics/sdk.md L1377-L1379 lists :always_on, :always_off, and :trace_based; minikube hardcodes :trace_based because it's the only branch that respects the wire-format input (parent span's W3C trace_flags). Same shape as Otel.Trace.Sampler's preserved :drop decision: a wire-format invariant remains after the user-policy filters are stripped.