chore: deprecate getRequestMetadata

[JustinBeckwith]

BREAKING CHANGE: The getRequestMetadata method has been deprecated on the IAM, OAuth2, JWT, and JWTAccess classes. The getRequestHeaders method should be used instead. The methods have a subtle difference: the getRequestMetadata method returns an object with a headers property, which contains the authorization header. The getRequestHeaders method simply returns the headers.

Old code

const client = await auth.getClient();
const res = await client.getRequestMetadata();
const headers = res.headers;

New code

const client = await auth.getClient();
const headers = await client.getRequestHeaders();

[codecov-io]

Codecov Report

Merging #414 into master will decrease coverage by 0.14%.
The diff coverage is 92.15%.

@@           Coverage Diff            @@
##           master   #414      +/-   ##
========================================
- Coverage   95.15%    95%   -0.15%     
========================================
  Files          15     15              
  Lines         970    981      +11     
  Branches      217    217              
========================================
+ Hits          923    932       +9     
- Misses         47     49       +2

Impacted Files	Coverage Δ
src/messages.ts	100% <100%> (ø)	⬆️
src/auth/iam.ts	100% <100%> (ø)	⬆️
src/auth/googleauth.ts	94.28% <100%> (ø)	⬆️
src/auth/oauth2client.ts	94.03% <80%> (-0.6%)	⬇️
src/auth/refreshclient.ts	90.47% <87.5%> (-0.23%)	⬇️
src/auth/jwtclient.ts	95.04% <90%> (ø)	⬆️
src/auth/jwtaccess.ts	98.24% <94.44%> (+0.06%)	⬆️

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[JustinBeckwith]

gentle ping @google/google-node-team @google/yoshi-nodejs