Make expression parser goroutine-safe.
Make expression parser goroutine-safe.

See https://github.com/prometheus/prometheus/issues/127

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etheus,tomwilkie/prometheus,dln/prometheus,prasincs/prometheus,fabxc/prometheus,jbdalido/prometheus,JavierZunzunegui/prometheus,tattsun/prometheus,AlekSi/prometheus,wndhydrnt/prometheus,adamron1/prometheus,gesellix/prometheus,wxdublin/prometheus,ton31337/prometheus