FR(tracing): add attribute-based sampling

Title: Attribute-based trace sampling

Description: In addition to the existing trace sampling configurations (client, random, and overall), it would be useful for Envoy to support some form of sampling based on request attributes. This would allow more flexible sampling logic, without the need for custom filters and shared metadata (for example).

This could work similar to the filters exposed by access logging (in particular, the CEL extension filter).

On the Istio effort, we've had customers/users ask for functionality that could be addressed by this feature, mainly at gateways and for new requests within a mesh. In particular, these requests have generally been of the following nature:

• sample new requests to certain services at 0% or 100% rate, while all other requests are sampled at the default random rate
• sample incoming requests for certain request paths at 0% or 100%, while all other requests are sampled at the default random rate

A sample proposal for what this might look like in the HCM.Tracing schema:

message Tracing {
    message AttributeSampling {
       message Sampler {
          enum SamplingDecision {
             ALWAYS_SAMPLE = 0;
             NEVER_SAMPLE = 1;
          }
          SamplingDecision decision = 1;
          repeated string expressions = 2;
       }
       repeated Sampler samplers = 1;
    }
    AttributeSampling attribute_sampling = 10;
    …
    type.v3.Percent client_sampling = 3;
    type.v3.Percent random_sampling = 4;
… 
}

It would operate with a semantic of:

• if a sampling decision has already been made, use that decision.
• if any sampler expression returns true, use the configured sampling decision for that sampler.
• if no sampler expression returns true, fallback to the randomSampling configuration for a sampling decision.

NOTE: this proposal does not support sampling at rates different than 0 or 100 for matches, in an attempt to maintain simplicity. It might be more flexible to replace the SamplingDecision decision with a type.v3.Percent rate field (at the marginal cost of more complex configuration and implementation).

This might lead to config like:

tracing:
  attributeSampling:
    samplers:
    - decision: NEVER_SAMPLE
      expressions:
      - request.url_path == '/no-trace'
      - request.host == 'metadata.internal.svc.cluster.local'
    - decision: ALWAYS_SAMPLE
      expressions:
      - request.url_path == '/login'
      - request.host == 'controller.admin.svc.cluster.local'
  randomSampling:
    value: 1.00

If this seemed reasonable and useful to the community, I'd volunteer to handle the implementation.