PMM-14153: Adding timestemp metrics for pbm backups

[theTibi]

PMM-14153 (optional, if ticket reported)

• Links to related pull requests (optional).

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Below we provide a basic checklist of things that would make it a good PR:

• Make sure to sign the CLA (Contributor License Agreement).
• Make sure all tests pass.
• Keep current with the target branch and fix conflicts if necessary.
• Update jira ticket description if necessary.
• Attach screenshots and/or console output to the jira ticket to confirm new behavior, if applicable.
• Leave notes to the reviewers if you need to focus their attention on something specific.

Once all checks pass and the code is ready for review, please add pmm-review-exporters team as the reviewer. That would assign people from the review team automatically. Report any issues on our Forum.

[it-percona-cla]

All committers have signed the CLA.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 65.29%. Comparing base (dc46ed5) to head (2bdf263).
Report is 110 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/pbm_collector.go	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1108      +/-   ##
==========================================
- Coverage   70.88%   65.29%   -5.60%     
==========================================
  Files          28       29       +1     
  Lines        3569     3083     -486     
==========================================
- Hits         2530     2013     -517     
- Misses        904      932      +28     
- Partials      135      138       +3     

Flag	Coverage Δ
agent	65.29% <0.00%> (-5.60%)	⬇️

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[theTibi]

I have tested it and it works, metric is created.