server: make the region heartbeat metric units correct

[nolouch]

What have you changed? (required)

In TiKV, we use seconds:

let now = SystemTime::now();
let ts = now.duration_since(UNIX_EPOCH).unwrap().as_secs();
req.set_timestamp(ts);

But in PD, we use milliseconds:

regionHeartbeatLatency.WithLabelValues(storeLabel).Observe(float64(time.Now().UnixNano()/int64(time.Millisecond)) - float64(request.GetTimestamp()))

So the heartbeat latency is wrong now.let the unit correct.

What are the type of the changes (required)?

• Bug fix (non-breaking change which fixes an issue)

Refer to a related PR or issue link (optional)

#1121

[disksing]

Need to update the metric bucket too. Also the tidb-ansible grafana template need to update too.

[disksing]

LGTM. PTAL @Connor1996

[Connor1996]

Is unit of second enough to distinguish latency?

[nolouch]

@Connor1996 we only need to know if the heartbeat latency is increasing, that to judge handle the heartbeat become slower.

[Connor1996]

LGTM