stats: when creating a new stat, extra symbol table locks are taken during tag extraction

I don't believe this is a serious performance issue because once a stat is created and placed in the central cache, this does not occur. But when a stat is initially created in a scope, the tag extraction process generates a new string that requires symbol-table lookup.

I have an in-progress solution to this I started quite some time ago and it's not compiling at this time: https://github.com/jmarantz/envoy/tree/tag-extract-without-symtab-lock -- I'm not sure it's worth the complexity it adds, but I wanted not to lose track of it.

An example of /stats/recentlookups output for a server we have in production illustrates the issue:
```
Count Lookup
90 vhost.vcluster.upstream_rq_retry
90 vhost.vcluster.upstream_rq_retry_limit_exceeded
90 vhost.vcluster.upstream_rq_retry_overflow
90 vhost.vcluster.upstream_rq_retry_success
90 vhost.vcluster.upstream_rq_timeout
90 vhost.vcluster.upstream_rq_total
105 host_REDACTED
525 _default
540 envoy.virtual_cluster
540 envoy.virtual_host
540 other

total: 15785
```
This server has been running for a long time so a total of 15k lookups spread over time is totally reasonable. ANd the numbers for (e.g. `envoy.virtual_cluster` grow slowly over time. But it would be nice if this display were clean and there were no repeated lookups of the same stat names.