add metric for ancient can't move slots

[jeffwashington]

Problem

When we are packing ancient append vecs, the current strategy is to not move accounts from a slot for all accounts with refcounts > 1. This is because we can't know easily what other slot(s) the older versions of the account are in. Thus, it isn't safe to move slots.

Summary of Changes

Add a metric to keep track of how many slots fit this criteria.

Fixes #

[HaoranYi]

lgtm

[codecov]

Codecov Report

Merging #33713 (87f8db3) into master (167dac2) will increase coverage by 0.0%.
Report is 4 commits behind head on master.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #33713   +/-   ##
=======================================
  Coverage    81.8%    81.8%           
=======================================
  Files         806      806           
  Lines      217867   217875    +8     
=======================================
+ Hits       178260   178307   +47     
+ Misses      39607    39568   -39     

[brooksprumo]

Thanks, lgtm!