Update dependencies in Matrix Federation Tester

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This involved doing some patchwork to get Prometheus working again, because they seem to have removed all the easy hatches.

We don't seem to use Prometheus on our setup for this service (I looked in the ansible)... but I thought it would be best to try and preserve the intent anyway.

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The absolutely massive go.sum is apparently operating as per design. I checked. I even did a go mod tidy but no further reductions.

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indeed, it really surprises me