Handle properly Google's tokeninfo 503/backend error

[clementdenis]

We are using Cloud Endpoints to serve a large scale API (millions of API requests per day).
We measure a low but significant amount of transient "503/backend error" when validating OAuth2 requests with GoogleAuth#getTokenInfoRemote, that translates into a "401/UnauthorizedException" for the API clients => this is not what one would expect for a transient exception.

This pull request introduce two main changes:

• It performs a single, immediate retry on https://www.googleapis.com/oauth2/v2/tokeninfo?access_token=XXX for IOExceptions and 5xx responses
• If the retry on IOException or 5xx fails, a "503/ServiceUnavailableException" is returned to API client, instead of a "401/UnauthorizedException"

[codecov-io]

Codecov Report

Merging #141 into master will increase coverage by 0.06%.
The diff coverage is 80.95%.

@@             Coverage Diff              @@
##             master     #141      +/-   ##
============================================
+ Coverage     79.95%   80.02%   +0.06%     
- Complexity     1673     1677       +4     
============================================
  Files           156      156              
  Lines          5573     5591      +18     
  Branches        725      729       +4     
============================================
+ Hits           4456     4474      +18     
+ Misses          841      840       -1     
- Partials        276      277       +1

Impacted Files	Coverage Δ	Complexity Δ
...le/api/server/spi/auth/EndpointsAuthenticator.java	100% <ø> (ø)	6 <0> (ø)	⬇️
.../server/spi/auth/GoogleAppEngineAuthenticator.java	85.45% <ø> (ø)	16 <0> (ø)	⬇️
...api/server/spi/auth/GoogleOAuth2Authenticator.java	87.09% <ø> (ø)	10 <0> (ø)	⬇️
...ava/com/google/api/server/spi/auth/GoogleAuth.java	85.71% <80.95%> (+0.96%)	33 <5> (+3)	⬆️
...rver/spi/response/ServiceUnavailableException.java	14.28% <0%> (+14.28%)	1% <0%> (+1%)	⬆️

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[tangiel]

Mostly LGTM, just two minor comments. Thanks for doing this!

[tangiel]

This should be indented four spaces in.

[tangiel]

Super minor nit but please add a space after the // here and below.

[clementdenis]

Fixed formatting (and too long lines as well). As there are only format change, I amended my commit and force pushed.

I'm coding with IntelliJ, can you confirm I should use Google's style guide?

[tangiel]

Yes, that should cover most of the formatting.