FR - More consistent emulator support

[saturnism]

Is your feature request related to a problem? Please describe.
I created examples for testing w/ emulators using testcontainers for datastore, firestore, pubsub, and spanner.

https://github.com/saturnism/testcontainers-gcloud-examples/tree/main/springboot

It took sometimes to figure out how to configure each starter to use the emulator correctly, mostly due to inconsistencies. This issue tracks the inconsistencies and proposed for a more consistent configuration for all emulators.

Starter	Emulator host	Host Format	Additional Properties
Datastore	spring.cloud.gcp.datastore.host	http://host:port
Firestore	spring.cloud.gcp.firestore.host-port	host:port	spring.cloud.gcp.firestore.emulator.enabled=true
Spanner	spring.cloud.gcp.spanner.emulator-host	host:port	spring.cloud.gcp.spanner.emulator.enabled=true
PubSub	spring.cloud.gcp.pubsub.emulator-host	host:port

Describe the solution you'd like
It'd be great to make all the emulator host configuration, host format, and additional properties requirement consistent.

Starter	Emulator host	Host Format	Additional Properties
Datastore	spring.cloud.gcp.datastore.emulator-host	host:port
Firestore	spring.cloud.gcp.firestore.emulator-host	host:port
Spanner	spring.cloud.gcp.spanner.emulator-host	host:port
PubSub	spring.cloud.gcp.pubsub.emulator-host	host:port

Or...

Starter	Emulator host	Host Format	Additional Properties
Datastore	spring.cloud.gcp.datastore.host-port	host:port	spring.cloud.gcp.datastore.emulator.enabled=true
Firestore	spring.cloud.gcp.firestore.host-port	host:port	spring.cloud.gcp.datastore.emulator.enabled=true
Spanner	spring.cloud.gcp.spanner.host-port	host:port	spring.cloud.gcp.datastore.emulator.enabled=true
PubSub	spring.cloud.gcp.pubsub.host-port	host:port	spring.cloud.gcp.datastore.emulator.enabled=true

This would conflict w/ the existing datastore emulator support, however. But that also means if we keep "emulator.enabled", it should only mean it if Spring Cloud GCP launches the emulator automatically (like Datastore)

However, I think the challenge is mostly in the underlying client libraries as well. There are some significant inconsistencies for client libraries regarding the use of EmualtorCredentials vs NoCredentials, where to configure plaintext for managed channel, and the host name, etc.

Describe alternatives you've considered
N/A

Additional context
https://github.com/saturnism/testcontainers-gcloud-examples/tree/main/springboot

[meltsufin]

Migrated to: GoogleCloudPlatform/spring-cloud-gcp#159