Add a report-only mode to the `BalanceCapacity` middleware
Add a report-only mode to the `BalanceCapacity` middleware

This adds a single environment variable that can be quickly set or
removed to toggle capacity enforcement. If this variable is set to any
value (including the empty string) then no requests will be rejected due
to load.

This will allow us to evolve the heuristics used in this middleware to
find the right balance before turning on enforcement.

This also increases the default logging percentage to a higher default.

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