Clarify usage of -instances_metadata fixes #259

[matalo33]

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[matalo33]

I signed it!

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CLAs look good, thanks!

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[broady]

that's pretty odd that you need to prefix with instance/attributes since the flag is called instances_metadata, but great catch. thank you.

[Carrotman42]

The "instances" in --instances_metadata refers to a Cloud SQL database instance, and is similar to the --instances flag.