fix for registering nested message types

[thiagodpf]

What?

client.load() (grpc module) was not registering protobuf nested messages in protoregistry.GlobalTypes.

Why?

It was necessary to go through the message structures starting from the root, but as it was before the PR, it only registered the root message.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

Closes #3160 issue.

backport from xk6-grpc PR

[CLAassistant]

All committers have signed the CLA.

[olegbespalov]

Thanks again for your contribution! 🚀

[codecov-commenter]

Codecov Report

Merging #3178 (aac34cd) into master (a2fe0a8) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head aac34cd differs from pull request most recent head 2bbfc1c. Consider uploading reports for the commit 2bbfc1c to get more accurate results

@@            Coverage Diff             @@
##           master    #3178      +/-   ##
==========================================
- Coverage   72.82%   72.76%   -0.07%     
==========================================
  Files         255      253       -2     
  Lines       19611    19611              
==========================================
- Hits        14281    14269      -12     
- Misses       4433     4441       +8     
- Partials      897      901       +4     

Flag	Coverage Δ
ubuntu	72.76% <100.00%> (-0.01%)	⬇️
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
js/modules/k6/grpc/client.go	85.00% <100.00%> (+0.31%)	⬆️

... and 10 files with indirect coverage changes