SpannerIO - Do not use .get() on ValueProvider during pipeline creation

[bvolpato]

Fixes issue #22146 (also reported on StackOverflow).

ValueProvider can be used to pass runtime parameters not available during pipeline creation (which is heavily used by Dataflow Templates).

In case a RuntimeValueProvider is being used, using .get() will throw the following exception:

Exception in thread "main" java.lang.IllegalStateException: Value only available at runtime, but accessed from a non-runtime context: RuntimeValueProvider{propertyName=spannerInstanceId, default=defaultinstanceid}
        at org.apache.beam.sdk.options.ValueProvider$RuntimeValueProvider.get(ValueProvider.java:254)
        at org.apache.beam.sdk.io.gcp.spanner.SpannerConfig.withInstanceId(SpannerConfig.java:165)
        at .... my code constructing SpannerConfig ...

So my changes adjust that behavior, to validate only if there is a reference to a provider, but not trying to use the value.
Ideally those can be validated elsewhere when not using the ValueProvider (the other entry methods do not specify @Nullable, so that should be handled already).

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[bvolpato]

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[bvolpato]

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[aromanenko-dev]

Thanks, LGTM. It would be great to update CHANGES.md to make this fix more visible in release notes.

Also, I was wondering why it was not caught by Spanner integration tests but, iinm, they are not running periodically on Jenkins like others. So, it would be great to add this as a separate issue/PR.

[bvolpato]

Thanks, LGTM. It would be great to update CHANGES.md to make this fix more visible in release notes.

Done, thank you. Please merge it if you agree with the CHANGES entry.

Also, I was wondering why it was not caught by Spanner integration tests but, iinm, they are not running periodically on Jenkins like others. So, it would be great to add this as a separate issue/PR.

From what I checked all tests are using the .withXyz(String), which end up instantiating .withXyz(StaticValueProvider), and then it works fine.

I think that the usage of .get() on a ValueProvider doesn't make sense as it kills the purpose of using providers. Maybe that could be enforced automatically (e.g., block any ValueProvider.get() outside a DoFn/PTransform) or at least pushed back on code review, but I am not sure how to proceed to improve the process here.

[bvolpato]

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