Metrics expiration is too aggressive

[gouthamve]

Beyla should expire metrics when the process goes away, but it is expiring them based on the last update. This is resulting in the following pattern:

This results in two issues:

1. rate function will return nothing during low traffic periods, while the correct answer is 0.
2. When there is infact a request, rate function will still return 0, because it calculates the delta, and there are no deltas in the graph above

[mariomac]

I'd suggest doing this task together or after #1087 , as #1087 involves some API changes that would require reworking once again this task.