fix default channelPoolSize to be 1 (instead of 0)

[igorbernstein2]

Fix a bug introduced in #436

[codecov-io]

Codecov Report

Merging #443 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #443      +/-   ##
============================================
+ Coverage      69.1%   69.11%   +0.01%     
  Complexity      635      635              
============================================
  Files           140      140              
  Lines          3013     3014       +1     
  Branches        230      230              
============================================
+ Hits           2082     2083       +1     
  Misses          843      843              
  Partials         88       88

Impacted Files	Coverage Δ	Complexity Δ
...api/gax/grpc/InstantiatingGrpcChannelProvider.java	40.17% <100%> (+0.51%)	8 <0> (ø)	⬇️

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[vam-google]

LGTM.
Please also add validation in InstantiatingGrpcChannelProvider() constructor before merging.

[igorbernstein2]

Validation already exists on the setter:
https://github.com/googleapis/gax-java/pull/443/files#diff-25ba434a47d99bcc0b998965a298661cR352

I don't have permissions to merge PRs. Please merge on my behalf

[vam-google]

Yes, the validation exists in the setter of the builder, but ideally class itself should be responsible for not allowing invalid initialization values, not the builder. It is a minor thing, so whatever, merging as is.