Document gRPC stram message's metadata

[olegbespalov]

What?

This PR does two things:

• Documents a new gRPC's stream message metadata object, which has been introduced in Add a second argument to callbacks for grpc streams with metadata k6#3801
• Cleans spotted unused constants in gRPC examples. I hope that works if I did this as a separate commit in this PR, if not I could extract this into a separate one

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/next folder of the documentation.

Related PR(s)/Issue(s)

grafana/k6#3801

[heitortsergent]

@olegbespalov thanks for opening the PR! I made some suggestions to make this more readable, let me know if it makes sense. 🙇