[Bug] deployment_sync_secs metrics still emitted for synced subgraphs

Bug report

We're still seeing deployment_sync_secs metrics still emitted for synced subgraphs on v0.31.0.

Specifically, we're still seeing these metrics emitted only when the indexer starts up.

I think this PR was intended to solve this in v0.31.0, but it doesn't seem it fully addresses the issue.

Relevant log output

No response

IPFS hash

No response

Subgraph name or link to explorer

No response

Some information to help us out

• Tick this box if this bug is caused by a regression found in the latest release.
• Tick this box if this bug is specific to the hosted service.
• I have searched the issue tracker to make sure this issue is not a duplicate.

OS information

None

まず PR 4489 を確認し、deployment_sync_secs が出力される indexer の起動経路を追跡します。すでに同期済みの subgraphs に対する動作を確認し、起動中にこのメトリクスが出力されなくなっていることを検証します。