all: update opencensus-proto to latest version and add grpc-gateway Metrics release is made

[odeke-em]

This issue is a placeholder to update to the opencensus-proto version that features Metrics for the grpc-gateway as per PR census-instrumentation/opencensus-proto#205

and then in the source code, we can add this

diff --git a/receiver/opencensusreceiver/opencensus.go b/receiver/opencensusreceiver/opencensus.go
index 1db22ab..abeba11 100644
--- a/receiver/opencensusreceiver/opencensus.go
+++ b/receiver/opencensusreceiver/opencensus.go
@@ -259,6 +259,11 @@ func (ocr *Receiver) startServer() error {
 				errChan <- err
 				return
 			}
+			err = agentmetricspb.RegisterMetricsServiceHandlerFromEndpoint(c, ocr.gatewayMux, endpoint, opts)
+			if err != nil {
+				errChan <- err
+				return
+			}
 
 			// Start the gRPC and HTTP/JSON (grpc-gateway) servers on the same port.
 			m := cmux.New(ocr.ln)

and then finally the grpc-gateway will be able to receive metrics in JSON form.

[odeke-em]

Kindly paging @songy23 @draffensperger