fix(metrics): Prevent underflow in connected_clients

[kishorviswanathan]

We have issues with the dragonfly_connected_clients under flowing and becoming a very huge value. I am not sure if this is the result of a race condition or an unbalanced number of increment/decrement calls. This is an attempt to add a check to prevent the underflow from happening.

[romange]

Does it happen often @kishorviswanathan ? Is it easily reproducible in your environment?

[kishorviswanathan]

@romange Yes, it happens quite often, but I don't know what is the reason for it or how to reproduce it.

[romange]

Can you please run your server with --migrate_connections=false and report if the issue persists?

[kishorviswanathan]

@romange I don't see any more connection count underflow after running with --migrate_connections=false. Is this like a know issue or something already fixed ?

[romange]

No it's not known. I just guessed where the bug might be. Feel free to open the issue and I won't accept this PR at this point as it fixes a symptom, not a core issue. Meanwhile - you have a workaround