Sampling rate can be updated without restarting application

[yjqg6666]

Which module is your feature request related to?
Agent

Is your feature request related to a problem?
No

Describe the solution you'd like
The updated sampling rate can not take effect without restarting. First make the samplingRate property in the SamplingRateSampler mutable, use the cluster feature to update the sampling rate.

ui -> web api -> collector -> agent.

A PR will be made.

Update: resolved by #8002.

[emeroad]

It seems to be a very useful feature.

However, it would be better to support other interface than the arthas.
What do you think about JMX support?

eg)

[yjqg6666]

If implemented by JMX, this part is still needed.

[yjqg6666]

I will append JMX control some time later.

[yjqg6666]

@emeroad Second thought on this, the cluster feature can be used to send a command to update sampling rate with/without TTL. The command can be triggered via collector API or Web GUI page and be sent to one agent or all the agents of a specified application. The existing communication channel can be used, the same way as sending thread-dumping command.