Sample logs consistently with trace sampling

[jeanbisutti]

The goal is to be able to sample logs consistently with trace sampling.

[jeanbisutti]

@jack-berg @jkwatson cc @trask

Would you agree to try to add this feature as experimental to this repository (perhaps with a different implementation)?

If the answer is yes, we could iterate in this repo:

• See how to allow users to enable this feature
• Add tests for log sampling
• Discuss about the implementation
• ...

Or would you recommend a different approach?

[jack-berg]

This whole thing needs to be configurable.

For an approach consistent with tracing (which I think is a good baseline to consider), we need a new LogSampler interface analagous to Sampler.

The key should sample method might have a contract like:

boolean shouldSample(
      Context context,
      Severity severity,
      Value<?> body,
      Attributes attributes);

Notes:

• Pass individual components of LogRecord instead of SdkReadWriteLogRecord so we can avoid allocations if shouldSample returns false. Also for consistency with tracing.
• Omit resource, scope arguments for consistency with tracing
• Omit timestamp, observed timesamp arguments for consistency with tracing
• Omit severityText because it mostly duplicates severity
• Return a boolean instead of the SamplingResult returned by tracing because log sampling is less complicated that trace sampling

With an interface like this, we could have built in samplers for common use cases. I.e. we could have a "trace based" sampler that retains logs if the span in context is sampled.