Hot restart support for user defined tags

After https://github.com/envoyproxy/envoy/pull/9743 we support specifying tags on metric creation, but there is no mechanism to populate the tags for metrics object after a hot restart. The full name (with the tag key-values included) would be transferred over, but without knowing how to split the name this would become the new stat name, and no tags would be moved over.

One possibility would be to insert a sentinel value in front of each key and value so that we can extract out the key/values during the stat merger:
```
stat.name.TAG_KEY.some.key.TAG_VALUE.some.value
```
would represent the stat `stat.name` with a single tag `some.key = some.value`

Another option would be to pass more than just a string during the hot restart, but iirc that would require changing the hot restart RPC proto, which might require bumping the hot restart version?

@jmarantz