proposal: cmd/trace: /prototrace endpoint

[aktau]

Context:

• The Catapult trace UI shipped with Go is not being maintained anymore (leading to issues like cmd/trace: requires HTML imports, which doesn't work on any major browser anymore #34374). The next generation developed by the Chrome team is the Perfetto UI (https://ui.perfetto.dev).
• One can manually load traces into https://ui.perfetto.dev. For this the JSON generated by cmd/trace is necessary. As discussed in cmd/trace: the trace command should not contain hardcoded javascript #16377, one can do go tool trace ... && curl .../jsontrace to get a file to upload.
• I didn't find a canonical issue for migrating to or supporting this web UI, so I'll file this issue that should make using the Perfetto UI easier.

Today, the perfetto UI produced a message I hadn't seen before:

Perfetto UI features are limited for JSON traces. We recommend recording proto-format traces from Chrome.

The docs appear to be: https://perfetto.dev/docs/reference/trace-packet-proto

The trace json output format is fairly verbose, so my hope is that by using proto format we produce more compact files that are easier to ship around and perhaps also lighter weight in the browser. Those extra features they're referencing may be nice too, but I don't know what they are. Currently we cap at 100MiB of JSON data. If we maintain the same cap on proto data (I don't know what the limits of the perfetto UI are) we could represent a larger timeline.

[prattmic]

I didn't find a canonical issue for migrating to or supporting this web UI, so I'll file this issue that should make using the Perfetto UI easier.

I thought we did have an issue for migrating to a new UI (likely Perfetto), but I can't find it. I was probably thinking of #34374 (comment).

Ironically, I just had an offline discussion the other day about how cmd/trace could potentially provide optional Perfetto output even if we haven't yet completely switched the default UI. We will eventually need to replace the Catapult UI, as it is effectively unmaintained and will eventually break again similar to #34374.

cc @golang/runtime @mknyszek @felixge @nsrip-dd