Review limit error metrics & error codes more generally

[mattheworiordan]

See https://support.ably.io/solution/articles/3000082364-explanation-all-metrics-within-rate-limit-errors. I've created this in response to a customer hitting a max channel rate limit, see https://support.ably.io/solution/articles/3000075165-is-there-a-maximum-number-of-channels-per-connection-.

@paddybyers @SimonWoolf can one of you please:

• [ ] Confirm that the metrics I have listed are correct. I had to guess the metric IDs.
• [ ] Please review https://support.ably.io/solution/folders/3000012310 to see if we have articles for the error codes relating to common rate limit issues. If there are any missing, please post them here and we can ask @tomczoink to help.

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