Erroring if gRPC parameters contains headers key

[olegbespalov]

What?

In #2371 we deprecated the headers param in favor of metadata.

k6/js/modules/k6/grpc/client.go

Lines 449 to 452 in 1892f91

	case "headers":
	c.vu.State().Logger.Warn("The headers property is deprecated, replace it with the metadata property, please.")
	fallthrough
	case "metadata":

Since it seems like a long time passed, we could finally remove the support of the headers.

And as the first step, we could start erroring if the headers param is presented.

Once this PR is merged, I'll open the PR with the far-away milestone to clean the error advising the use of metadata.

Why?

We should clean up the headers

Changelog

Minor breaking change: Previously, gRPC invoke's headers param was deprecated and fell through to the metadata. Since this version of k6, using the headers param will result in an error. Instead, users should use the metadata param.

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)

#3343 (comment)

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.06% ⚠️

Comparison is base (1892f91) 72.94% compared to head (daa77bb) 72.89%.

❗ Current head daa77bb differs from pull request most recent head 1ddd358. Consider uploading reports for the commit 1ddd358 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3350      +/-   ##
==========================================
- Coverage   72.94%   72.89%   -0.06%     
==========================================
  Files         261      259       -2     
  Lines       20018    20012       -6     
==========================================
- Hits        14603    14587      -16     
- Misses       4488     4493       +5     
- Partials      927      932       +5     

Flag	Coverage Δ
ubuntu	72.89% <0.00%> (-0.01%)	⬇️
windows	?

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

Files Changed	Coverage Δ
js/modules/k6/grpc/client.go	82.74% <0.00%> (-0.64%)	⬇️

... and 8 files with indirect coverage changes

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