fix: prometheus metrics expiration must be set for each metric

[shreemaan-abhishek]

Description

Previously a feature was added that enabled making prometheus metrics expirable. Such configuration didn't have the flexibility to be set for only certain metrics. This PR ensures each metric (among status, latency and bandwidth) has configurable expiry.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)