Add last backup time to prometheus metrics

[DrGonzo]

For monitoring and alerts like time() - resticprofile_backup_time

[creativeprojects]

It looks good to me 😄

For consistency, would you mind renaming the metric to time_seconds? it's kind-of a convention

Thanks 👍🏻

[codecov[bot]]

Codecov Report

Merging #132 (e95ebbb) into master (5a9c952) will increase coverage by 1.64%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   68.19%   69.83%   +1.64%     
==========================================
  Files          68       74       +6     
  Lines        7307     7559     +252     
==========================================
+ Hits         4983     5279     +296     
+ Misses       2105     2068      -37     
+ Partials      219      212       -7     

Impacted Files	Coverage Δ
monitor/prom/backup.go	100.00% <100.00%> (ø)
monitor/prom/metrics.go	94.20% <100.00%> (+0.17%)	:arrow_up:
config/schedule_config.go	59.61% <0.00%> (-14.71%)	:arrow_down:
shell_command.go	88.60% <0.00%> (-2.57%)	:arrow_down:
monitor/hook/sender.go	76.66% <0.00%> (-2.50%)	:arrow_down:
commands.go	47.57% <0.00%> (-0.32%)	:arrow_down:
schedule/handler_darwin.go	48.06% <0.00%> (-0.18%)	:arrow_down:
schedule/job.go	59.67% <0.00%> (ø)
config/global.go	85.00% <0.00%> (ø)
schedule/handler.go	100.00% <0.00%> (ø)
... and 24 more

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[DrGonzo]

dont mind, thanks