add HealthEndpoints map as public

[sergeylanzman]

add public HealthEndpoints for set custom Health urls

[rakyll]

We unfortunately need to finalize census-instrumentation/opencensus-specs#151 before throwing new APIs to this problem.

[semistrict]

@sergeylanzman could you explain a bit what prompted this change? what problem are you experiencing?

[sergeylanzman]

@Ramonza
Sometimes application have health check on custom url not only "/healthz". Sometime health check intergrade from internal framework and more complicate(include check connection to DB, important services and etc). I think it's not relevant for tracing and need "clean" from tracing data/statistics.
This PR add support options for add/replace other options for health check for delete from tracing data

[sergeylanzman]

I think better solution create method "SetUntraceableEndpoints" and set internal map with endpoints.

[semistrict]

What about having a per-path trace.Sampler as a map[string]trace.Sampler field on ochttp.Handler?

[rakyll]

Can we achieve the blacklisting functionality via #869 (comment)?

We can return a NeverSample for health endpoints.

[rakyll]

Once #924 is merged, I will create a ochttp/ochttputil package with some filter implementations.