add protobuf test

Author: zeitlinger

exporters/prometheus/src/test/java/io/opentelemetry/exporter/prometheus/PrometheusHttpServerTest.java

@@ -239,6 +239,24 @@ void fetchOpenMetrics() {
                 + "# EOF\n");
   }
 
+  @Test
+  void fetchProtobuf() {
+    AggregatedHttpResponse response =
+        client
+            .execute(
+                RequestHeaders.of(
+                    HttpMethod.GET,
+                    "/metrics",
+                    HttpHeaderNames.ACCEPT,
+                    "Accept: application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily"))
+            .aggregate()
+            .join();
+    assertThat(response.status()).isEqualTo(HttpStatus.OK);
+    assertThat(response.headers().get(HttpHeaderNames.CONTENT_TYPE))
+        .isEqualTo("application/vnd.google.protobuf; proto=io.prometheus.client.MetricFamily; encoding=delimited");
+    // don't decode the protobuf, just verify it doesn't throw an exception
+  }
+
   @SuppressWarnings("ConcatenationWithEmptyString")
   @Test
   void fetchFiltered() {