Fix state reference in MetricCollection

[SkafteNicki]

What does this PR do?

Fixes #1051
When users call .items(), .values or __getitem__ on a metric collection that have enabled compute groups, the metric states are currently only by referenced, leading to weird results sometimes when metrics in the collection is individually interacted with (as when logging in PL with self.log).
PR fixes this by creating copies (instead of references) when needed.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1076 (5abcedd) into master (65594f1) will decrease coverage by 4%.
The diff coverage is 96%.

@@           Coverage Diff           @@
##           master   #1076    +/-   ##
=======================================
- Coverage      95%     90%    -4%     
=======================================
  Files         181     181            
  Lines        8085    8103    +18     
=======================================
- Hits         7654    7315   -339     
- Misses        431     788   +357