Fix schema of Bots

Description

This MR includes a small fix for the last commit (606b6d23) by adding the metering-service-client in the schema of Bots similar to (https://gitlab.com/BuildGrid/buildgrid/-/blob/7859f3db0eb0f2b0422cc5a781a3a09aa475876d/buildgrid/_app/settings/schemas/services/execution.yaml#L75).

Note that a Bots instance can be created in 2 ways: 1) implicitly in Execution 2) explicitly in Bots. Therefore, the attribute should be allowed at both places.