Remove reflection to get App Engine credentials and project ID

[ajkannan]

In production App Engine (standard runtime), reflection to get project ID and credentials won't work unless the user depends on appengine-api-1.0-sdk, even though the necessary jars are already provided by the runtime. To get around this, we should declare a "provided"-scoped dependency on that library in gcloud-java-core and use the AppIdentityService directly instead of through reflection to get credentials and the project ID.

[ajkannan]

Update: this won't fix the issue. The SDK dependency is used to set whether the APIs are available on app engine or not in the app.yaml generated for deployment. Thus, when using App Engine (standard environment), users should include the SDK dependency in their pom.xml files.

I'll keep this issue open because we can still implement the suggestion above for minor performance improvements.

[ajkannan]

Closing this as the benefits are quite small compared to the effort required. Any CPU performance benefit from avoiding reflection is dwarfed by the time it takes for the subsequent RPC call to complete.