Input metadata Match

[andremoah]

First of all, thank you for this awesome package.

I have been using this for some time and was able to work around every problem found, but there is something i can't seem to discover which is metadata matching.

In the above sample, i wan't to match 2 requests: 1- Metadata with Culture: 'pt-PT' 2- Empty Metadata

The problem with this is that grpc-mock will always return the first stream instead of throwing the error provided in the second array element. I saw that metadata is implemented for error output responses but not for input.

This is something important for Sad Path tests, i could try to submit a PR, but first i would like to know if this is already possible somehow.

Thanks in advance.