add prometheus metrics to record warning total and last status of backups

[allenxu404]

Thank you for contributing to Velero!

Please add a summary of your change

• add metrics backup_warning_total for record of total warnings
• add metrics backup_last_status for record of last status of the backup

Does your change fix a particular issue?

Fixes #5380 #5398

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov-commenter]

Codecov Report

Merging #5779 (a5a165b) into main (598333d) will decrease coverage by 0.05%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##             main    #5779      +/-   ##
==========================================
- Coverage   40.67%   40.62%   -0.05%     
==========================================
  Files         243      244       +1     
  Lines       21035    21263     +228     
==========================================
+ Hits         8555     8638      +83     
- Misses      11857    12000     +143     
- Partials      623      625       +2     

Impacted Files	Coverage Δ
pkg/controller/backup_controller.go	55.64% <37.50%> (-0.20%)	⬇️
pkg/persistence/object_store.go	53.09% <0.00%> (-3.10%)	⬇️
pkg/podvolume/restorer.go	9.09% <0.00%> (-2.48%)	⬇️
pkg/plugin/framework/action_resolver.go	5.00% <0.00%> (-0.76%)	⬇️
pkg/util/kube/pod.go	0.00% <0.00%> (ø)
pkg/plugin/framework/server.go	0.00% <0.00%> (ø)
pkg/podvolume/restorer_factory.go	0.00% <0.00%> (ø)
pkg/plugin/framework/common/plugin_kinds.go	0.00% <0.00%> (ø)
...reitemaction/v2/restartable_restore_item_action.go	67.10% <0.00%> (ø)
pkg/plugin/clientmgmt/process/client_builder.go	94.59% <0.00%> (+0.15%)	⬆️
... and 2 more

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