Update aws-prereqs.rst

[dmitchsplunk]

Requirements

• [ X ] Content follows Splunk guidelines for style and formatting.
• [ X ] You are contributing original content.

Describe the change

While working with a customer I discovered that the API Gateway charts weren't lighting up as expected. This is because the Method, Resource, and Stage dimensions were not being included with metrics such as Count, Latency, and 4XXError. The solution was to enabled detailed CloudWatch metrics for each API Gateway stage of interest, as discussed in Dimensions for metrics.

Please let me know if you believe this should be noted in another area of the docs.

[sheilattran8]

@aurbiztondo-splunk Heads up about this ticket, which is related to https://splunk.atlassian.net/browse/SWAT-8433. (I believe that you own the GDI docs, but feel free to tag anyone else if needed)

[aurbiztondo-splunk]

Thanks for the update, @dmitchsplunk!