pkg/config: support seconds in pprof configuration

[alperkokmen]

this updates the definition of PprofProfilingConfig such that a new integer field Seconds is added which can be set for profiles, in an attempt to resolve https://github.com/parca-dev/parca/issues/2818.

additionally, it changes the behavior where setting any profiling configurations will end up overriding the default so merge will now become a replace.

seconds is accepted as a query parameter and with this change, when provided, it will be used as scraping duration rather than the scraping interval value. this allows having scraping where process being profiled isn't under constant profiling. this only applies to configurations that have delta enabled.

https://pkg.go.dev/net/http/pprof

[alperkokmen]

@brancz i have taken a stab at implementing the proposed solution in #2818. please take a look.

[alperkokmen]

@brancz, i just wanted to bump this up to get some eyes on it; could you please take a look for add folks who would be able to review? thank you.

[brancz]

Apologies for missing this, this looks great!

[brancz]

I'm really sorry, this needs a rebase, but otherwise lgtm! And sorry again for this slipping through the cracks, so thank you for pinging me again. In the future also feel free to ping me on the Parca Discord server if you want to make sure to get a faster review! :)

[alperkokmen]

thanks for taking a look, @brancz. i rebased and addressed the feedback. PTAL when you get a chance.

[alperkokmen]

@brancz i think i need one last approval before checks are executed?

[brancz]

Thank you for the awesome contribution!