[bugfix] Let prometheus client do its own compression handling

[tsmethurst]

Description

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Update routing to not pass our gzip middleware to prometheus http endpoint, but let it do its own compression handling. Because of this change in this PR, prometheus overwrites the content-encoding header to identity (ie., no compression) even if our gzip middleware compresses the body, which was confusing http clients (understandably). Rather than fighting prometheus and trying to hack around this by setting another content-encoding later or something, it's easier to just let prometheus win this round and do its own compression.

Tested this on the testrig and it fixes the issue.

closes #3216

Checklist

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