Adds metadata-flavor header to metadata server ping for compute engine

[freddd]

Fixes #282

[codecov]

Codecov Report

Merging #283 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #283      +/-   ##
============================================
+ Coverage     78.21%   78.24%   +0.03%     
  Complexity      330      330              
============================================
  Files            21       21              
  Lines          1446     1448       +2     
  Branches        157      158       +1     
============================================
+ Hits           1131     1133       +2     
  Misses          237      237              
  Partials         78       78

Impacted Files	Coverage Δ	Complexity Δ
...m/google/auth/oauth2/ComputeEngineCredentials.java	86.33% <100%> (+0.09%)	31 <0> (ø)	⬇️
...tp/java/com/google/auth/oauth2/UserAuthorizer.java	76.57% <0%> (+0.13%)	20% <0%> (ø)	⬇️

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[chingor13]

Thank you!