This adds metrics & pprof to the op-challenger. I had to add a default config
option to the metrics & pprof in order to make the tests work (because it
verifies that the result of the CLI parsing is as expected when compared to
an config struct which is constructed).

This also had to modify how the challenger stores its version information
because we include that information in the metrics when we record that it is
up. Unlike the proposer & batcher, the challenger has a different initialization
flow which does not easily make the version information accessible.