Add cost metrics for all providers

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/415944

Add cost metrics for all providers

• measures token size for Antorhopic/Vertex requests/responses
• prompt/response size divided by estimated token size is used so this measurement may not be very precise, but it should be still useful for overall statistics
• also stores snowplow events for tokens used for single AI requests and summary of all requests done by the client

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Before	After

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