Envoy Gateway - support tracing sampler configuration

[jan-mrm]

Description:
I'd like to be able to configure the "tracing sampler" in the Envoy Gateway.

Currently only the samplingRate is configurable:

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyProxy
spec:
  telemetry:
    tracing:
      samplingRate: 100
...

The current default sampler behaves like / is ParentBased(root=TraceIdRatioBased), so the incoming headers trace sampling decision is respected (forwarded) and if non is available (so when no traceparent header exists) then its sampled by the defined samplingRate.

Since there are other samplers / other sampling behaviors it would be nice to be able to choose / configure this as the user who is handling the EnvoyProxy / Envoy Gateway config.

Other samplers do exist already in envoy, see [2,] but its a work-in-progress extension and is not configurable yet (also not configurable with a workaround by using an EnvoyPatchPolicy, see [1]).

I could imagine a config like spec.telemetry.tracing.sampler that could take enum values AlwaysOn / AlwaysOff / ParentBased / TraceIdRatioBased / ParentBasedAlwaysOn / ParentBasedTraceIdRatioBased / ...
The samplingRate would then only have an effect if the sampler is RatioBased.

Relevant Links:

• [1] previous discussion - Envoy Gateway - configure telemetry tracing sampler? #8436
• [2] OpenTelemetry Samplers within Envoy in Extensions 'HTTP tracers' - https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/trace/opentelemetry/samplers
• [3] OpenTelemetry Sampler spec - https://opentelemetry.io/docs/specs/otel/trace/sdk/#sampler

[codefromthecrypt]

@zhaohuabing @zirain @arkodg fyi I will take this as I realize envoy tracing isn't integrated into aigw, and if it is, the sampling decision needs to move from extproc to envoy (as it is in front of extproc). So, this means I also need this to work. I'm not a project member of gateway, yet, but consider me assigned!

[zirain]

@codefromthecrypt assign this to you, thanks!

[codefromthecrypt]

#8529