Do not try to ask for a bogus distribution

[lukesneeringer]

The autogen code within GAX has an issue where it believes that gapic-google-longrunning-v1 is a real package, even though it is actually being vendored inside of GAX (unlike everything else). That makes the code that tries to add the gapic/x.y.z portion of the metrics header fail out.

For the moment, this code should be removed, as it raises an exception. Ultimately (when it is not a Saturday), OperationsClient should be given the value of the GAPIC that instantiates it.

Alternatively, we could split this out into a separate package for consistency, and track its version.

This PR:

• Fixes PR #166 breaks LRO usage #170
• Fixes Speech throwing DistributionNotFound error google-cloud-python#3097

[geigerj]

Please file an issue to fix the underlying issue (i.e., either in the code generator, or by separating this package out)

[lukesneeringer]

Done.