refactor: metadata supports key corresponding to multiple values

[ibrahimalbarghouthi]

Adding Add/Values func for transport and metadata layers, fixed tests a… #2771

[codecov-commenter]

Codecov Report

Merging #2772 (d43d31e) into main (f03f5f8) will decrease coverage by 0.29%.
The diff coverage is 69.49%.

❗ Current head d43d31e differs from pull request most recent head 4f808fe. Consider uploading reports for the commit 4f808fe to get more accurate results

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@@            Coverage Diff             @@
##             main    #2772      +/-   ##
==========================================
- Coverage   84.75%   84.47%   -0.29%     
==========================================
  Files          88       88              
  Lines        3891     3923      +32     
==========================================
+ Hits         3298     3314      +16     
- Misses        419      434      +15     
- Partials      174      175       +1     

Impacted Files	Coverage Δ
transport/grpc/transport.go	85.71% <0.00%> (-14.29%)	⬇️
transport/http/transport.go	75.00% <0.00%> (-9.38%)	⬇️
transport/transport.go	100.00% <ø> (ø)
log/helper.go	74.57% <66.66%> (-7.06%)	⬇️
metadata/metadata.go	96.36% <87.50%> (-3.64%)	⬇️
middleware/metadata/metadata.go	92.59% <100.00%> (+0.59%)	⬆️

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[tonybase]

LGTM

[tonybase]

@ibrahimalbarghouthi please fix lint

[ibrahimalbarghouthi]

@tonybase Done please merge

[shenqidebaozi]

LGTM