Add support for snapshot metrics

[patelsmit32123]

Description

#165 Following the issue opened by ginkel, this PR adds the required changes for basic support for snapshot related metrics, starting with snapshot_age. More snapshot related metrics can be added on top of this changes.

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• All my commits include DCO.
  DCO stands for Developer Certificate of Origin and it is your declaration that your contribution is correctly attributed and licensed. Please read more about how to attach DCO to your commits here (spoiler alert: in most cases it is as simple as using -s option when doing git commit).
  Please be aware that commits without DCO will cause failure of PR CI workflow and can not be merged.

[lukas-vlcek]

FYI, to get rid of the Developer Certificate of Origin (DCO) check failure you need to sign all commits with -s option (git commit -s ...).

This will add a specific line into the commit message containing your signature with your email. See any other commit message in this repository for example of the text. You can create the signature manually in the commit message as well but using -s is more systematic way.

(Use of git -s option assume you configured git like this git config user.email "[email protected]" first.)

[patelsmit32123]

@lukas-vlcek signed off the commit, please review

[patelsmit32123]

@lukas-vlcek I have added integration test and updated the README also, PTAL

[patelsmit32123]

@lukas-vlcek can you please help review this PR?

[lukas-vlcek]

@patelsmit32123 Sorry, I was on 🌴 last week. I will do it this week.

[lukas-vlcek]

@patelsmit32123 Thanks a lot for your patience! I am really sorry for the delay in processing this.

• First of all, I am almost done with reviewing it and I already made some comments on your PR before, can you see them? (The review is not submitted yet so I do not know if GitHub shows my comments to you in this case)
• Second, a new release 2.17.0.0 has been cut today, would you mind rebasing the PR?

[patelsmit32123]

@lukas-vlcek i am not able to see your comments

[lukas-vlcek]

Thanks for this PR. I am proposing some changes and enhancements. Let me know if anything is not clear.
Overall, great contribution, thanks for the effort.

[lukas-vlcek]

@patelsmit32123 Can you see my comments now?

[patelsmit32123]

@lukas-vlcek thanks for the review, resolved all comments

[patelsmit32123]

@lukas-vlcek is there anything else needed to be done for this PR?

[lukas-vlcek]

@patelsmit32123 I think it is ready. I am trying to implement #315 now (your PR will benefit from it) but if a new release is needed before BWC tests are ready I will just go ahead and merge it to make it part of the next release.
Does it sound good?

[lukas-vlcek]

LGTM