trace: make maximum active traces configurable

[nvanbenschoten]

This commit adds a "max_active" query param to net/trace's /debug/requests
HTTP handler. When provided, the parameter configures the maximum number of
active traces to show. This allows users to override the default max value
of 20, which was previously a constant value enforced by maxActiveTraces.

[gopherbot]

This PR (HEAD: e0d3da4) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/net/+/222675 to see it.

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[gopherbot]

Message from Nathan VanBenschoten:

Patch Set 1:

Friendly ping. It doesn't look like there's a current owner of /x/net/trace so I'm not sure who to route this change to for review. It's a small but potentially high-impact improvement to observability.

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[nvanbenschoten]

@bradfitz any chance you'd be willing to give this small patch a review? The engineers at CockroachDB have found that this change improves the utility of net/trace's /debug/requests page significantly for debugging distributed deadlocks.