router: make shadow requests inherit parent sampling decision by default

[agrawroh]

Description

This PR changes the behavior of shadow request sampling so that if trace sampling is not explicitly configured in the shadow policy, the shadow request will inherit the parent's sampling decision.

After this change, sampling will follow the trace sampling policy of the original request, which prevents oversampling when runtime sampling is disabled.

Fixes: #37766

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Commit Message: router: make shadow requests inherit parent sampling decision by default
Additional Description:
Risk Level: Low
Testing: Added Unit Tests
Docs Changes: Added
Release Notes: Added

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @wbpcode
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #37874 was opened by agrawroh.

see: more, trace.

[wbpcode]

Thanks for take this. And some comments are added to start the work.

[wbpcode]

/wait

[repokitteh-read-only]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

🐱

Caused by: #37874 was synchronize by agrawroh.

see: more, trace.

[nulltrope]

@wbpcode Not sure if this is something that is normally allowed but I work with @samschlegel (original issue reporter) and while this change is undoubtably useful for everyone as implemented, unfortunately the xDS control plane we are using (GCP Traffic Director) hardcodes the shadow policy's trace_sampled to true and they are not very quick to make changes to their API surface, if at all.

As a workaround in the meantime, would it be possible to add a second runtime flag to basically "force" ignoring the shadow policy's trace_sampled config (so essentially an extra if condition here ), or alternatively, a runtime flag similar to the existing tracing runtime flags that controls only shadow policy/mirrored tracing %?

Totally understand if the answer is "no" given we have a unique use case here with our control plane choice, but figured I would get a pulse on whether it could be considered :)