Add label select for k8s metadata processor

[llhhbc]

Description

Add label selector for k8s metadata processor. For only need watch some pod by label selector. I'm not sure if I should limit which pods to fetch only in origin.

Related Issue

Motivation and Context

How Has This Been Tested?

[dxsup]

Could you please also provide an example of the configuration for selecting specified pods? It seems like you are using a set of "pod's labels" to match the "DataGroup's labels" which contain no k8s metadata when consumed by this processor.

[llhhbc]

I just ues the struct of kubernetes, It's configable. I will add some examples.

[llhhbc]

@dxsup I have add some docs

[llhhbc]

@dxsup Can this be merged?

[dxsup]

Sorry for the late reply. I guess the intention of this PR is to filter the DataGroups that match the labels given in the configuration file. A better implementation is we introduce a new processor which is designed to finish only this feature. How do you think? Could you spare time to finish this feature as a new processor?

[llhhbc]

ok