https://s3.amazonaws.com/lyft-envoy/coverage/report-master/coverage.html shows a bunch of relatively new functionality in stats lacks coverage. A common pattern is that due to multiple implementations of an API, we don't test-cover all cases.

Summarizing:

1. histogram_impl.h: used() not called, 1 missing line
2. isolated_store_impl.h: no coverage for findCounter(), findGauge, or findHistogram(). ~10 lines
3. metric_impl.h: used() not called, 1 missing line
4. scope_prefixer.h: 1 line, const symbolTable() accessor not called, 1 line.
5. stat_data_allocator_impl.h: latch(), cacheSHouldImport(), setShouldImport() not called: 3 lines
6. store_impl.h: counterFromStatName() etc not called: ~6 lines
7. thread_local_store.h: const symbolTable() accessor not called. as well as not-found return-statement for findCounter() etc, 6 lines.

I'd like to suggest this division of labor:

@ahedberg can you do isolated_store_impl.h -- may want to outline the impls too?
@fredlas can you do stat_data_allocator_impl.h ?

I'll do the rest?