## Description 

Because we run snapshot upload and garbage collection as a background
task but we perform rocksdb checkpointing at end of epoch
unconditionally for configured nodes, any failure in either the upload
or garbage collection path will lead to an accumulation of old db
checkpoints, which will inevitably lead to disk filling and make things
more difficult to debug.

This PR adds a metric to periodically count the number of rocksdb
checkpoints that exist on local disk. Except in rare cases (backfills),
this number should generally be lower than 3, so we can add alerting on
this for early intervention.

## Test plan 

👀 

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## Release notes

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- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API: