-help output does not mention all flags about pprof

[aalexand]

@rauls5382 I think it's a reasonable request that these flags should display in the -help message, especially when pprof is invoked as part of golang. I think the fix is to move these from being flags to being pprof variables in internal/driver/commands.go?

[rauls5382]

These are compatibility options from the old pprof. They are convenient aliases for -sample_index=xxx, to be used on some specific profile types. We already have a generalized handling of sample_index so that we can say "inuse_space" as a pprof command for the right kind of profiles.

I propose to just add some text to the help files describing these legacy options in terms of the new options. I'll send a PR.