Add OcAgentOptions override for service_name. (#391)

* Don't set service name to Hostname.

Other languages' exporters don't do this either.

Author: g-easy

opencensus/exporters/trace/ocagent/internal/ocagent_exporter.cc

@@ -310,7 +310,9 @@ void Handler::InitNode() {
   library_info->set_core_library_version(OPENCENSUS_VERSION);
 
   auto service_info = nodeInfo_.mutable_service_info();
-  service_info->set_name(::opencensus::common::Hostname());
+  if (!opts_.service_name.empty()) {
+    service_info->set_name(opts_.service_name);
+  }
 }
 
 void Handler::ConnectAgent() {

opencensus/exporters/trace/ocagent/ocagent_exporter.h

@@ -33,6 +33,9 @@ struct OcAgentOptions {
   // The RPC deadline to use when exporting to OcAgent.
   absl::Duration rpc_deadline = absl::Seconds(5);
 
+  // (optional) If not empty, set the service name to this.
+  std::string service_name;
+
   // (optional) By default, the exporter connects to OcAgent using address. If
   // this stub is non-null, the exporter will use this stub to send
   // gRPC calls instead and ignore the address. Useful for testing.