Use FLAGS_heartbeat_rpc_timeout_ms for heartbeater's timeout

instead of hardcoding 10sec timeout #2418

Summary:
Currently, the TS uses a hard-coded 10 sec timeout for the heartbeat RPC.
If the TServer has a lot of tablets, the initial RPC reporting the full tablet report can take a long time.

Use FLAGS_heartbeat_rpc_timeout_ms to make this configurable for such large clusters.

Test Plan: eyeball

Reviewers: kannan, mihnea, hector

Reviewed By: hector

Subscribers: ybase, bogdan

Differential Revision: https://phabricator.dev.yugabyte.com/D7279

src/yb/tserver/heartbeater.cc

@@ -64,10 +64,12 @@ using namespace std::literals;
 DEFINE_int32(heartbeat_rpc_timeout_ms, 15000,
              "Timeout used for the TS->Master heartbeat RPCs.");
 TAG_FLAG(heartbeat_rpc_timeout_ms, advanced);
+TAG_FLAG(heartbeat_rpc_timeout_ms, runtime);
 
 DEFINE_int32(heartbeat_interval_ms, 1000,
              "Interval at which the TS heartbeats to the master.");
 TAG_FLAG(heartbeat_interval_ms, advanced);
+TAG_FLAG(heartbeat_interval_ms, runtime);
 
 DEFINE_int32(heartbeat_max_failures_before_backoff, 3,
              "Maximum number of consecutive heartbeat failures until the "
@@ -435,7 +437,7 @@ Status Heartbeater::Thread::TryHeartbeat() {
   }
 
   RpcController rpc;
-  rpc.set_timeout(MonoDelta::FromSeconds(10));
+  rpc.set_timeout(MonoDelta::FromMilliseconds(FLAGS_heartbeat_rpc_timeout_ms));
 
   req.set_config_index(server_->GetCurrentMasterIndex());