x/perf/benchstat: add -metrics=speed,alloc/op flag

[nigeltao]

benchstat reports a number of metrics:

• alloc/op
• speed
• time/GC
• time/op

In my automated reports, I'm only interested in a subset of those, even if the underlying source files provide more than that.

Would you (@aclements) accept a patch so that I can say benchstat -metrics=speed,alloc/op?

Right now, I can workaround it with piping benchstat's output through sed. It's not that onerous, but if others might find it useful, it's possibly a little cleaner to do it in benchstat itself.

[bcmills]

CC @josharian

[josharian]

Sounds fine to me, but I think this is Austin's call.

[gopherbot]

Change https://golang.org/cl/309969 mentions this issue: cmd/benchstat: new version of benchstat