wip(metric-issues): Update script to create metric issues

[snigdhas]

Adding to bin/load-mocks to generate metric issues for local testing.
Uses:
bin/load-mocks --load-metric-issues --metric_alert_id={alert_id} - this will create a metric issue for the given metric alert rule
bin/load-mocks --load-metric-issues - this will create a metric alert rule and then create a metric issue for that rule

using the --load-metric-issues flag will skip any of the other setup that bin/load-mocks usually handles.

[codecov]

Codecov Report

Attention: Patch coverage is 19.23077% with 21 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/utils/mockdata/core.py	19.23%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #83025      +/-   ##
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+ Coverage   87.55%   87.63%   +0.07%     
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  Files        9411     9423      +12     
  Lines      536904   537012     +108     
  Branches    21113    21113              
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+ Hits       470107   470603     +496     
+ Misses      66431    66043     -388     
  Partials      366      366