metrics: explicitly set Accept header (#4774)

If a request is made to the Pomerium metrics endpoint with an Accept
header requesting the Prometheus protobuf exposition format, some
metrics will be missing from the response.

These missing metrics are obtained by replaying the incoming request to 
an OpenCensus metrics exporter. This exporter honors the request for the
protobuf format, however Pomerium expects this response to be in the 
text format.

We can avoid this mismatch by explicitly requesting the text format from
the OpenCensus exporter, regardless of the incoming request's Accept
header.

(Note: the Pomerium metrics endpoint always responds with text format 
metrics, even if the protobuf format is requested.)

internal/telemetry/metrics/providers.go

@@ -196,6 +196,8 @@ func ocExport(name string, exporter *ocprom.Exporter, r *http.Request, labels []
 	return func(context.Context) promProducerResult {
 		// Ensure we don't get entangled with compression from ocprom
 		r.Header.Del("Accept-Encoding")
+		// Request metrics in text format.
+		r.Header.Set("Accept", "text/plain")
 
 		rec := httptest.NewRecorder()
 		exporter.ServeHTTP(rec, r)