add plevel to metric_value table + migrations

[peterdudfield]

Pull Request

Description

• Add p level to metric values table
• migrations

How Has This Been Tested?

CI tests

• [x] Yes

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

Merging #246 (8e18467) into main (f627fca) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #246   +/-   ##
=======================================
  Coverage   95.26%   95.26%           
=======================================
  Files          27       27           
  Lines        1731     1732    +1     
=======================================
+ Hits         1649     1650    +1     
  Misses         82       82           

Files	Coverage Δ
nowcasting_datamodel/models/metric.py	97.22% <100.00%> (+0.03%)	:arrow_up:

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