Add pcommon.Map helper to add a key to the map if does not exists

[bogdandrutu]

This new API is intended to be similar with "LoadOrStore" from the sync.Map and a helper to avoid iterating over the map (lookup) twice for when we need to only insert a key if it does not exists.

Example https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/internal/coreinternal/attraction/attraction.go#L307:

if _, found = attrs.Get(action.Key); found {
	continue
}
av.CopyTo(attrs.PutEmpty(action.Key))

We lookup key in Get as well as PutEmpty, and this can be re-written as:

if val, found = attrs.GetOrPutEmpty(action.Key); !found {
	av.CopyTo(val)
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.68%. Comparing base (1169338) to head (349e0e1).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14023      +/-   ##
==========================================
+ Coverage   91.66%   91.68%   +0.02%     
==========================================
  Files         654      654              
  Lines       42626    42633       +7     
==========================================
+ Hits        39072    39087      +15     
+ Misses       2741     2735       -6     
+ Partials      813      811       -2     

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[dmitryax]

Hm.. The new chlogen validation doesn't work for this case. pkg/pdata would make it, not ideal though

Can you please add some coverage as well?