Tags/Labels friendly stats system for Envoy

*Title*: *Tags-friendly stats system for Envoy*

*Description*:

Here is a further discussion of #20220. Envoy provides powerful stats system which is well designed and carefully optimised. This system provides first-class support for statsd. Regular expression-based extraction of tags and output in prometheus format were added later.

The current method of using regular expressions to extract tags is very flexible and powerful. But if there are some irregular identifiers (such as including variable length dots), the regular expression will be very complicated. And the regular expression for extracting tags can only be configured through bootstrap.

So may be we can try to do some enhancements to current stats system to get better support to the tags/labels stats.

This PR is used to record the disccusion.

[optional *Relevant Links*:]

Related comments: https://github.com/envoyproxy/envoy/issues/20220#issuecomment-1061986614

Initial idea about the enhancement: https://docs.google.com/document/d/1Y5bHKcwiFE_cueH3obYfQ59RCwz9688ZC9mNGOYW_hM/edit?usp=sharing