Exclude cloudprober from distribution

[bytestream]

Hi

I noticed that this repository is installed when using google/cloud-translate packages and other google/* packages. It's listed as a dependency of google/gax

What is cloudprober? Does it need to be present in a composer distribution?

It looks to me like the following files can excluded via .gitattributes:

• third_party/googleapis/
• cloudprober/
• doc/

I can open a PR if this is OK.