Allow customization of health endpoints in ochttp server

This is part feature request and part bug. The automatic tracing includes a check where both the ochttp client and server components inspect the URL path before attaching a trace to the request being made. Unfortunately those paths to skip tracing are hardcoded:

opencensus-go/plugin/ochttp/trace.go

Lines 231 to 238 in 6531013

    
           func isHealthEndpoint(path string) bool { 
        
           	// Health checking is pretty frequent and 
        
           	// traces collected for health endpoints 
        
           	// can be extremely noisy and expensive. 
        
           	// Disable canonical health checking endpoints 
        
           	// like /healthz and /_ah/health for now. 
        
           	if path == "/healthz" || path == "/_ah/health" { 
        
           		return true

called from the server here:

opencensus-go/plugin/ochttp/server.go

Lines 89 to 91 in 6531013

    
           func (h *Handler) startTrace(w http.ResponseWriter, r *http.Request) (*http.Request, func()) { 
        
           	if isHealthEndpoint(r.URL.Path) { 
        
           		return r, func() {}

Is your feature request related to a problem? Please describe.
The /health and /metrics endpoints in our server end up being sampled alongside service endpoints and are generating excessive traces.

Describe the solution you'd like
Enhance ochttp.Handler to allow callers to supply a set of paths or path pattern to use for unconditionally skipping sampling. This could either be something like a HealthEndpoints []string field for directly specifying paths, a HealthEndpointPattern *regex.Regex for use with Match/MatchString. The default paths currently in use could be used as the default to provide backwards compatibility.

While a similar enhancement could also be introduced for the client this issue is focused on improving the server-side sampling logic.

Describe alternatives you've considered
Exposing health/metrics/etc endpoints through a different http.Server instance is possible and could be used to bypass ochttp entirely but would have to listen on a different port and introduce the possibility of unexpected or divergent behavior from the state of the service's primary http.Server (e.g. during server Shutdown or config reload).

Additional context
I could find time to work on a PR for this soon but would greatly appreciate if a maintainer could weigh in with a preferred approach and a general suggestion for what kinds of unit tests are expected. Perhaps a new unit test is desired or it might be sufficient to adds a few case to this test (or both):

opencensus-go/plugin/ochttp/trace_test.go

Lines 204 to 210 in 3da91ae

    
           func TestEndToEnd(t *testing.T) { 
        
           	tc := []struct { 
        
           		name            string 
        
           		handler         *Handler 
        
           		transport       *Transport 
        
           		wantSameTraceID bool 
        
           		wantLinks       bool // expect a link between client and server span

	func isHealthEndpoint(path string) bool {
	// Health checking is pretty frequent and
	// traces collected for health endpoints
	// can be extremely noisy and expensive.
	// Disable canonical health checking endpoints
	// like /healthz and /_ah/health for now.
	if path == "/healthz" \|\| path == "/_ah/health" {
	return true

	func (h Handler) startTrace(w http.ResponseWriter, r http.Request) (*http.Request, func()) {
	if isHealthEndpoint(r.URL.Path) {
	return r, func() {}

	func TestEndToEnd(t *testing.T) {
	tc := []struct {
	name string
	handler *Handler
	transport *Transport
	wantSameTraceID bool
	wantLinks bool // expect a link between client and server span