🐛 Do not set up local mini-front-proxy without additional mappings file

[embik]

Summary

It looks like we missed a conditional in d8cbc0a and now every kcp setup that starts sets up the mini-front-proxy handlers. Looking at the PR (#3199), I don't think that was the intention, and it also started exposing an unauthenticated /metrics endpoint on the kcp server binary.

My quick read of the code / logic is: Only register the extra handlers if we indeed want to start a mini-front-proxy by passing additional mappings, but I'm not sure if this is the way to go. @mjudeikis please advise if we should do it differently, but we need to make sure this development feature doesn't bleed into kcp-the-server.

Related issue(s)

Fixes #3360

Release Notes

Stop exposing mini-front-proxy handlers (including `/metrics`) on kcp server unless `--additional-mappings-file` is passed

[embik]

/retest

flake?

[mjudeikis]

/lgtm
/approve

We can revisit this flow if we need to later.

[kcp-ci-bot]

LGTM label has been added.

Git tree hash: 8b97dd61b7eb1492b902b757844c9bded360a415

[kcp-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mjudeikis

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[embik]

/retest

[embik]

/cherry-pick release-0.27

[kcp-ci-bot]

@embik: new pull request created: #3362

In response to this:

/cherry-pick release-0.27

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