Actually I've tracked this a bit, and it happens (though silently) even when using one of the built-in example files.
Steps to reproduce:
1) open file google-ads-php/vendor/grpc/grpc/src/lib/BaseStub.php
2) place a breakpoint on line 47
3) open file google-ads-php/examples/Reporting/GetKeywordStats.php
4) for testing purposes, insert valid customer ID on line 42
5) run the GetKeywordStats.php file with a debugger enabled
6) once it hits your breakpoint on line 47 of BaseStub.php, step into the code
7) notice that the code attempts to find and load ChannelCredentials.php, but fails to do so
8) notice also that a call to ChannelCredentials on line 53 does nothing, as the class previously failed to load
Subsequent requests seem to complete successfully, so is this something I should just ignore? And if not, how can I configure my system to actually find ChannelCredentials.php?