Signed-off-by: Joshua Marantz <jmarantz@google.com>

Commit Message: Rolls back the rollback PR #20835 , re-enabling fast admin stats, now that #20855 has landed.
Additional Description:
This brings the performance back to this state:
```
------------------------------------------------------------------
Benchmark                        Time             CPU   Iterations
------------------------------------------------------------------
BM_AllCountersText             467 ms          466 ms            2
BM_UsedCountersText           37.0 ms         37.0 ms           19
BM_FilteredCountersText       1793 ms         1792 ms            1
BM_AllCountersJson             504 ms          504 ms            1
BM_UsedCountersJson           37.2 ms         37.2 ms           19
BM_FilteredCountersJson       1839 ms         1839 ms            1
```
So: around half a second of CPU burst for 1M json & text stats, rather than 1.8 seconds for text and 4.7 seconds for json. We still have a std::regex bottleneck when a filter is specified.
Risk Level: medium -- this re-enables calling of code that previously had had races, though #20855 repro'd and fixes them.
Testing: //test/...
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Signed-off-by: Joshua Marantz <jmarantz@google.com>

Commit Message: Fix Stats::Scope destruct/iterate race by holding onto a weak_ptr<Scope> the scopes_ hash-table in ThreadLocalStore. Also adds GUARDED_BY thread annotation to the scopes_ hash table and refactors a bit to ensure thread safety across all accesses. The thread-safety analysis needs more-than-usual annotation assistance for two reasons:
 * the analysis system does not see that `ThreadLocalStoreImpl::lock_` and `ThreadLocalStoreImpl::ScopeImpl::parent_.lock_` are the same.
 * in safeMakeStat call-sites, for code-sharing reasons, we need to take a reference to the guarded `central_cache_` entry before we decide whether we need to take the lock the protects it, so we need to disable analysis in that case. This way we can share the code that finds stats in the TLS-cache without taking locks.

A couple of helper methods,`centralCacheLockHeld()` and `centralCacheNoThreadAnalysis()`, were added to allow analysis to run with minimally scoped annotations.

A testcase was added which duplicates the race between looping over the stats for admin, and creating/destroying scopes, using the fast /stats implementation that was disconnected in prod in envoyproxy#20835. This PR leaves the fast implementation disconnected, but fixes it. A separate PR will roll back envoyproxy#20835 after this lands.

Additional Description: The repro can spot the race by reverting the definition of `ThreadLocalStoreImpl::forEachScope` to to its prior state, taking into account that `scopes_` is now a `map<ScopeImpl*, weak_ptr<ScopeImpl>` rather than a `set<ScopeImpl>`.
```
  for (auto iter : scopes_) {
    f_scope(*(iter.first));
  }
```
 Then test/server/admin:stats_handler_test, test will fail wtih
```
 RUN      ] ThreadedTest.Threaded
terminate called after throwing an instance of 'std::bad_weak_ptr'
  what():  bad_weak_ptr
```
Risk Level: low -- scope iteration is being fixed here, but that doesn't happen in production yet.
Testing: //test/...
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Signed-off-by: Joshua Marantz <jmarantz@google.com>

Commit Message: Rolls back the rollback PR envoyproxy#20835 , re-enabling fast admin stats, now that envoyproxy#20855 has landed.
Additional Description:
This brings the performance back to this state:
```
------------------------------------------------------------------
Benchmark                        Time             CPU   Iterations
------------------------------------------------------------------
BM_AllCountersText             467 ms          466 ms            2
BM_UsedCountersText           37.0 ms         37.0 ms           19
BM_FilteredCountersText       1793 ms         1792 ms            1
BM_AllCountersJson             504 ms          504 ms            1
BM_UsedCountersJson           37.2 ms         37.2 ms           19
BM_FilteredCountersJson       1839 ms         1839 ms            1
```
So: around half a second of CPU burst for 1M json & text stats, rather than 1.8 seconds for text and 4.7 seconds for json. We still have a std::regex bottleneck when a filter is specified.
Risk Level: medium -- this re-enables calling of code that previously had had races, though envoyproxy#20855 repro'd and fixes them.
Testing: //test/...
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Signed-off-by: Joshua Marantz <jmarantz@google.com>

Commit Message: Rolls back the rollback PR envoyproxy/envoy#20835 , re-enabling fast admin stats, now that envoyproxy/envoy#20855 has landed.
Additional Description:
This brings the performance back to this state:
```
------------------------------------------------------------------
Benchmark                        Time             CPU   Iterations
------------------------------------------------------------------
BM_AllCountersText             467 ms          466 ms            2
BM_UsedCountersText           37.0 ms         37.0 ms           19
BM_FilteredCountersText       1793 ms         1792 ms            1
BM_AllCountersJson             504 ms          504 ms            1
BM_UsedCountersJson           37.2 ms         37.2 ms           19
BM_FilteredCountersJson       1839 ms         1839 ms            1
```
So: around half a second of CPU burst for 1M json & text stats, rather than 1.8 seconds for text and 4.7 seconds for json. We still have a std::regex bottleneck when a filter is specified.
Risk Level: medium -- this re-enables calling of code that previously had had races, though #20855 repro'd and fixes them.
Testing: //test/...
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Signed-off-by: Joshua Marantz <jmarantz@google.com>