update labkit dependency and add pprof server

This updates the labkit dependency to a more recent version. There's been a bunch of changes since the last update.

$ git log --format=oneline --merges 3253d7975ca7...f06e28fff6fa

f06e28fff6fa5abb2002dde598310ced2d57ffcd Merge branch '5-upgrade-to-sentry-go' into 'master'
fac94cb428e67efc019e7ba67429b53fa9b394c5 Merge branch 'osw-avoid-misleading-success-log' into 'master'
839779ccd150da52511e5951c08020c165c7fdc7 Merge branch 'osw-rename-serve-function' into 'master'
b4bc994715ef922a3c09f753f0ae97d0d37e92dc Merge branch 'osw-pass-config-to-profiler' into 'master'
b34ac27984514151d4d3c1b27441e6aa15caef8e Merge branch 'access-log-combined-latency' into 'master'
9cb7801f7feafecc53f5a061ffeae5c11567da9b Merge branch 'pokstad1-export-correlation' into 'master'
4f8041c014c2e9c6eed42674d76f5235afb5a36a Merge branch 'osw-anticipate-profiler-initialization' into 'master'
2631448819e649f939f2025ff66acefe630364c7 Merge branch 'osw-add-go-stackdriver-profiler' into 'master'
bd6bebe139947617f072ee6b1c47e5e150e940c2 Merge branch 'run-go-mod-tidy' into 'master'
a462057a6414bf97ef2584e8af72debefa2d259a Merge branch 'go-1.13' into 'master'
1b744917b4df4c97e57447ed2423c6a70d834ec9 Merge branch 'fix-gitaly-compat' into 'master'

I also went ahead and added a call to monitoring.Start(), which enables pprof.

This would have been really useful when investigating gitlab-com/gl-infra/production#1999 (closed).