tracing: General mechanism to avoid tracing health check requests

The envoy proxy has provided a mechanism to avoid tracing health check requests for some time. The current mechanism was implemented in https://github.com/envoyproxy/envoy/pull/3578#event-1684409739.

The problem with the current approach is that the decision to not sample a health check request is made after the span is already created. This is because the decision is applied when processing the request in the health check filter.

Not all tracer implementations are able to handle changing the sampling decision after the span has already been created. We almost ran into a problem with this when the datadog tracer https://github.com/envoyproxy/envoy/issues/5504 was added, but a workaround was found. However we now have this issue with the OpenCensus tracer: https://github.com/envoyproxy/envoy/pull/5387.

A possible solution has been described in https://github.com/envoyproxy/envoy/issues/5504#issuecomment-452612825

cc @cgilmour @g-easy @kyessenov