Add more metrics to monitor the sessions from distinct components

## Enhancement

We can add some options to `Pool`, for example:

```
s, err := pool.Get(WithByComp("TTL"))
```

And we can know the session is obtained by the component `TTL` and then the internal metrics counter with name "TTL" increased by 1.

When the session is closed or put back, the TTL counter decreased by 1.