Fix autoscaler - was unnecessarily forcing to a single metric

[nat-henderson]

Multiple metrics are supported, as the new test proves. Also, we weren't successfully storing the right information about custom metrics on read... you can imagine why we didn't catch that since it's a little-used feature.

This fixes #38.

[rosbo]

Remove this debug line

[nat-henderson]

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccComputeAutoscaler -timeout 120m
=== RUN   TestAccComputeAutoscaler_basic
=== RUN   TestAccComputeAutoscaler_update
=== RUN   TestAccComputeAutoscaler_multicondition
--- PASS: TestAccComputeAutoscaler_basic (69.14s)
--- PASS: TestAccComputeAutoscaler_multicondition (70.74s)
--- PASS: TestAccComputeAutoscaler_update (82.94s)
PASS
ok      github.com/terraform-providers/terraform-provider-google/google 82.948s

[nat-henderson]

Oh, and since CI's still behind:

01:04 $ make
==> Checking that code complies with gofmt requirements...
go install

[ghost]

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