Add flag for block profiling

[jml]

Fixes #1672

Test by adding --app.block.profile.rate=1, running scope in
multi-tenant mode and browsing to http:///debug/pprof/

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This change is

[2opremio]

Test by adding --app.block.profile.rate=1

Isn't this flag a bit fine-grained? How about simply enabling block profiling with a default value when supplying --debug?

Also, could you please update site/building.md?

[2opremio]

Another option is adding something like --debug.block-profiling-rate=n which would affect both in the app and the probe.

(these are just suggestions, it's ultimately up to you)

[2opremio]

Thinking about it a bit more, in the documentation it seems we separate debugging (showing more info in the logs) from profiling so feel free to ignore my comments about the flag names.

[jml]

Isn't this flag a bit fine-grained? How about simply enabling block profiling with a default value when supplying --debug

I anticipate that we'll get the default value wrong & will want to iterate quickly. Flags are the best way we currently have of doing that.

Another option is adding something like --debug.block-profiling-rate=n which would affect both in the app and the probe.

Having one flag for both app & probe seems like a good idea. Does your other comment about separating debug from profiling preclude this as a flag name?

[2opremio]

Does your other comment about separating debug from profiling preclude this as a flag name?

Probably, at least we separate them in the docs.

[2opremio]

LGTM (provided you get a green from circle)