receiver/opencensus/ocmetrics: ignore io.EOF on JSON upload

When the grpc-gateway was added to this receiver/opencensus
Trace receiver in PR #270, we recognized that browsers
and other sources that send JSON in one burst won't stream
so would encounter an io.EOF. This PR makes the metrics
receiver replicates that behavior.

We'll get it this PR also ported over to the OpenTelemtry
service.

Fixes #610

Author: odeke-em

receiver/opencensusreceiver/ocmetrics/opencensus.go

@@ -17,6 +17,7 @@ package ocmetrics
 import (
 	"context"
 	"errors"
+	"io"
 	"time"
 
 	"google.golang.org/api/support/bundler"
@@ -110,6 +111,11 @@ func (ocr *Receiver) Export(mes agentmetricspb.MetricsService_ExportServer) erro
 
 		recv, err = mes.Recv()
 		if err != nil {
+			if err == io.EOF {
+				// Do not return EOF as an error so that grpc-gateway calls get an empty
+				// response with HTTP status code 200 rather than a 500 error with EOF.
+				return nil
+			}
 			return err
 		}
 	}