Allow configuring whether thread renames should be respected or make perfmark more scalable

[aktau]

We'd like to trace execution of RPCs on our frontend using perfmark.

For debugging purposes, our frontend renames a thread when work is scheduled on it. When the task starts executing, it looks at certain important fields and puts it in the thread name. This is to aid debugging when the process crashes. It allows trivially identifying which requests were running when the server died.

Right now, perfmark creates a new "row" whenever this happens. Sadly, this soon leads to an inability to load the traces in the browser as there will be too many threads.

A couple of options:

• A configuration option to make perfmark ignore thread renames somehow.
• A configuration option (or trace display option) to limit the history of traces. I believe that in any given 1 minute window there's not likely to have been too many threads active.
• Something else...?