Create relay-meter-service.md

[adshmh]

This is a proposal for the first version of relay-meter functionality being delivered by TxDB service.

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• T-19745 Remove rate-limiter's dependency on InfluxDB (unlink task)

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[adshmh]

Link T-19745

[rem1niscence]

One think to take into account is that the tx db service does not hold specific info regarding rate limiting, it holds data of all relays which then is aggregated to data warehouse in multiple ways, one of them being available for rate limiting. What I mean is that tx db service does not have special data structures specific for rate limiting, so all the code for this will be made in a way that this is only presented as a reporting service, which then can be consumed by services like rate limiter