grpc-native-core: do not use deprecated getRequestMetadata

[alexander-fenster]

Fixes #545.

Since getRequestMetadata is now deprecated in google-auth-library, we are getting deprecation warnings in Google Cloud client libraries.

Since there is no good way to tell if the user passed old or new googleCredentials object, we can check if it has getRequestHeaders method or not, and if it has one, use it, otherwise fallback to the old getRequestMetadata. Ugly but works.

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[alexander-fenster]

I signed it

[alexander-fenster]

As discussed today, please release the patch release with this change if possible. Since google-gax requires "grpc": "^1.12.2", the change will be picked up by client libraries automatically. Thanks!

[nicolasnoble]

We'll have to cherry-pick it for 1.15.

[JustinBeckwith]

Thanks folks. When do you expect 1.15 to land?

[nicolasnoble]

We'll likely do a 1.15.1 tomorrow since this is the only change.