do not increment sync operation count metric on per index set

[sandeepsukhani]

What this PR does / why we need it:
Do not increment index sync operation count metric from index set since we already track it on the overall sync operation in table manager here.

This would also fix unexpected increases in failure counts due to stale index list cache which we handle already by checking if the error is errIndexListCacheTooStale and retry the sync.

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%