Metrics for session numbers

[wey-gu]

Is your feature request related to a problem? Please describe.
One of our users asked me about this, which reminded me that, we should have this observability on session numbers to avoid it accumulated or raised in an unexpected trend.

Describe the solution you'd like
Provide such metrics.

Describe alternatives you've considered
Or could be queried via ngql?

Additional context
As mentioned in first section.
Talking with @cangchen8180 .

[cangchen8180]

每个graphd当前active的session数（有clientip 创建时间等信息更好）；
服务器建立过的session总数；
主动过期的session个数

session的上限；

session占用的资源情况

[CPWstatic]

Now you can call show sessions to get the total sessions.