simplifies and makes clearer all vault prometheus metrics

[fcgravalos]

Added

• New secrets_manager_vault_max_token_ttl metric, so a user could alert based on this and secrets_manager_token_ttl

Fixed

• Deprecates secrets_manager_vault_token_expired metric as it was quite confusing since it's not really possible for secrets-manager to actually not it's expired, just "close to expire".
• Renames counter metrics to follow the Prometheus naming standard with the _total suffix instead of _count.
• Simplifies prometheus token renewal metrics by merging secrets_manager_vault_token_lookup_errors_count and secrets_manager_vault_token_renew_errors_count into one single metric secrets_manager_vault_token_renewal_errors_total with one more dimension called vault_operation which will be one of lookup-self, renew-self, is-renewable.

[codecov-io]

Codecov Report

Merging #21 into release-0.2.0-rc.2 will increase coverage by 0.98%. The diff coverage is 75%.

@@                  Coverage Diff                  @@
##           release-0.2.0-rc.2     #21      +/-   ##
=====================================================
+ Coverage               82.61%   83.6%   +0.98%     
=====================================================
  Files                      11      11              
  Lines                     443     427      -16     
=====================================================
- Hits                      366     357       -9     
+ Misses                     60      55       -5     
+ Partials                   17      15       -2

Impacted Files	Coverage Δ
backend/vault_metrics.go	100% <100%> (+5.26%)	:arrow_up:
backend/vault.go	66.66% <50%> (+1.86%)	:arrow_up:

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[fcgravalos]

Rebased, though I don't think it's needed, we could have just merged later on into the release branch.