Add token expiry metrics

[t4niwa]

Description

Add token_expires_in_seconds metrics

example 1

If default is set for TOKEN_EXPIRY , the max_expiry or min_expiry won't appear

$ curl -sfS localhost:9793/metrics | grep 'token_expires_in_seconds'
# HELP token_expires_in_seconds Indicates remaining time until the token expires.
# TYPE token_expires_in_seconds gauge
token_expires_in_seconds{domain="domain1",role="role1",type="roletoken"} 14120.64295
token_expires_in_seconds{domain="domain1",role="role1",type="accesstoken"} 12754.642959
token_expires_in_seconds{domain="domain2",role="role2",type="roletoken"} 12754.642952

example 2

$ curl -sfS localhost:9793/metrics | grep 'token_expires_in_seconds'
# HELP token_expires_in_seconds Indicates remaining time until the token expires.
# TYPE token_expires_in_seconds gauge
token_expires_in_seconds{domain="domain",role="users",type="roletoken"} 13877.364748
token_expires_in_seconds{domain="domain",max_expiry="10000",role="role",type="roletoken"} 9510.364777
token_expires_in_seconds{domain="domain",min_expiry="10000",role="role",type="roletoken"} 13923.364768
token_expires_in_seconds{domain="domain",max_expiry="100000",role="role",type="roletoken"} 99486.364787
token_expires_in_seconds{domain="domain",max_expiry="10000",min_expiry="10000",role="role",type="roletoken"} 9560.364757

Example 3 (If number is big)

below example is 30 days long tokens

$ curl -sfS localhost:9793/metrics | grep 'token_expires_in_seconds'
# HELP token_expires_in_seconds Indicates remaining time until the token expires
# TYPE token_expires_in_seconds gauge
token_expires_in_seconds{domain="domain",min_expiry="10000",role="role",type="roletoken"}  2.591979418962684e+06
token_expires_in_seconds{domain="domain",min_expiry="10000",role="role",type="accesstoken"}  2.591979418947666e+06

Type of change

• [ ] Bug fix
• [x] New feature
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Non-code changes (update documentation, pipeline, etc.)

Flags

• [ ] Breaks backward compatibility
• [ ] Requires a documentation update
• [ ] Has untestable code

---

Checklist

• [x] Followed the guidelines in the CONTRIBUTING document
• [x] Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• [x] Tested and linted the code
• [x] Commented the code
• [x] Made corresponding changes to the documentation
• [x] Passed all pipeline checking

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip]
• Delete the branch after merge