metrics: promote etcd_debugging_mvcc put_total and delete_total

[hexfusion]

This PR promotes the following experimental metrics to stable.

• etcd_debugging_mvcc_put_total -> etcd_mvcc_put_total
• etcd_debugging_mvcc_delete_total -> etcd_mvcc_delete_total

Fixes #10950

/cc @p0lyn0mial

[codecov-io]

Codecov Report

Merging #10962 into master will increase coverage by 0.77%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10962      +/-   ##
==========================================
+ Coverage   64.21%   64.98%   +0.77%     
==========================================
  Files         401      401              
  Lines       37473    38222     +749     
==========================================
+ Hits        24062    24838     +776     
+ Misses      11804    11730      -74     
- Partials     1607     1654      +47

Impacted Files	Coverage Δ
mvcc/metrics.go	91.66% <ø> (ø)	⬆️
mvcc/metrics_txn.go	100% <100%> (ø)	⬆️
pkg/tlsutil/tlsutil.go	86.2% <0%> (-6.9%)	⬇️
pkg/fileutil/purge.go	65.9% <0%> (-6.82%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
pkg/mock/mockserver/mockserver.go	72.72% <0%> (-2.28%)	⬇️
clientv3/balancer/balancer.go	86.36% <0%> (-2.28%)	⬇️
mvcc/watchable_store.go	83.5% <0%> (-1.76%)	⬇️
embed/etcd.go	68.5% <0%> (+0.39%)	⬆️
etcdserver/v3_server.go	72.43% <0%> (+0.45%)	⬆️
... and 23 more

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[gyuho]

@jingyih Txn metrics would combine those changes (ranges + puts + deletes)

[gyuho]

This PR is missing register calls for putCounterDebug. Let me send a PR to fix.