fix: bug where we could error after writing a status code

[TylerHelmuth]

Which problem is this PR solving?

• WriteOtlpHttpResponse did a no-no where it could return an error after writing a status code to the Writer. This makes it really hard for users to know the state of their Writer once the call to WriteOtlpHttpResponse ends.

Short description of the changes

• Update function order to no longer error once we've written a status code.
• Updates the comments.

[kentquirk]

I almost caught this in the previous review. I was looking at example code and noticed that even the Go authors never looked at the error value from Write...but didn't think it through all the way.

Good catch!