metrics: fix unexpected usage output shown on cli

[crazy-max]

What I did

Got a hard time to pinpoint this issue but finally found it comes from the cli proxy while parsing args with pflags for build metrics:

$ docker buildx inspect --help

Usage:  docker buildx inspect [NAME]

Inspect current builder instance

Options:
      --bootstrap        Ensure builder has booted before inspecting
      --builder string   Override the configured builder instance
Usage of buildx:
      --builder string

Now with flag set usage silenced:

$ docker buildx inspect --help

Usage:  docker buildx inspect [NAME]

Inspect current builder instance

Options:
      --bootstrap        Ensure builder has booted before inspecting
      --builder string   Override the configured builder instance

cc @tonistiigi

Related issue

• cli usage output looks wrong with builder persistent flag docker/buildx#990

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[glours]

LGTM

[crazy-max]

Ok to merge this one @glours @mat007 @gtardif?