ext_proc: GRPC tracing requests are always sampled regardless of the parent decision

[cainelli]

Title: ext_proc GRPC tracing requests are always sampled regardless of the parent decision

Description: Essentially the exact same issue and possible fix seen in Lua httpCall (#33200) and ext_authz (#19343).

When introduced sampling support in ext_proc I overlooked this and can open a PR to fix it. I just would like to agree on the approach, I see some basic options.

1. add .setSampled(absl::nullopt) in the StreamOptions (as we do in Lua httpCall and ext_authz)
2. Change the default sample behavior as once considered to avoid the same issue in different places.

[phlax]

cc @tyxia @wbpcode

[phlax]

note: envoy maintainers will be mostly on vacation until 6th jan