resource/aws_appautoscaling_policy: Support additional predefined metric types in validation

[bflad]

Closes #3112

Validation of the attributes for aws_appautoscaling_policy has been a mixed bag of requests for the validation at plan time and issues like this where the validation is not up to date. Ideally, we would perform some type of compile time reflection of these applicationautoscaling.MetricType* constants instead, but this at least brings us up to all the currently supported predefined metrics types and adds unit testing.

make test TEST=./aws TESTARGS='-run=TestValidateAppautoscalingPredefinedMetricSpecification'
==> Checking that code complies with gofmt requirements...
go test ./aws -timeout=30s -parallel=4
ok  	github.com/terraform-providers/terraform-provider-aws/aws	1.800s

[radeksimko]

LGTM, but I'd probably prefer a more future-proof solution as mentioned in my comment.

I'll leave it up to you though.

[radeksimko]

Something tells me this list is going to continue to grow and we just shouldn't be validating it at all, instead of playing catch up with Amazon... 🤔

[bflad]

I agree with you for the most part. Its a challenge of better user experience (plan-time validation and requiring provider updates) vs development time (SDK not providing a quick way to get the full list of them). Since this PR is approved and we're planning on releasing today, I'll bring this one in as-is to at least fix the current state of the world, then will followup with a PR to remove the validation completely. We'll see if it lands in time for today as well.

As a separate pet project I will likely try to automatically go generate these from the SDK as I personally have found the plan-time validation to helpful and this type of generation would be applicable in a few other places. Then again I also always kept my AWS provider up to date in my environments.

[bflad]

As noted, followup PR to remove validation (if we want to): #3141

[bflad]

This has been released in terraform-provider-aws version 1.8.0. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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