send "group by" in export requests.

[umanwizard]

Add the "group by" field to the query for pprof download and pprof.me export. This makes it so the selected label set will be available in the exported data.

[umanwizard]

BTW, the labels still don't show up in pprof.me, but they do show up in the pprof you can download from pprof.me. So I suspect there is another issue on the pprof.me side that needs to be fixed.

[alwaysmeticulous]

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_{Expected differences? Click here. Last updated for commit 0eeb036. This comment will update as new commits are pushed.}

[brancz]

Just need to fix that lint, then this lgtm and we can have a separate look at pprof.me, but I suspect we just don't pass the group by labels through correctly in the pprof.me backend.