chore: move baggage methods in propagation namespace

[vmarchaud]

Since we moved the span context method into trace namespace, i'm also proposing to move [create/get/set]Baggage into the propagation namespace.

We still export baggageEntryMetadataFromString which feels kinda wrong to me since it doesn't do anything apart creating a unusued object, could we have a less strict type for the metadata so we don't need this function ?

[codecov]

Codecov Report

Merging #55 (9268926) into main (0a3bee4) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   94.62%   94.66%   +0.04%     
==========================================
  Files          40       40              
  Lines         502      506       +4     
  Branches       80       80              
==========================================
+ Hits          475      479       +4     
  Misses         27       27              

Impacted Files	Coverage Δ
src/api/propagation.ts	100.00% <100.00%> (ø)
src/index.ts	100.00% <100.00%> (ø)

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[obecny]

Because the methods will be available under propagation namespace instead of global, i think you should add info to Upgrade Guidelines, other than that lgtm

[dyladan]

I agree with bart you should update the upgrade guidelines. Other than that LGTM so I will approve

[vmarchaud]

@obecny I rebased + added the upgrade guideline, PTAL

[obecny]

lgtm