Previously, we used prometheus.DefaultRegisterer to register our stats, which uses global state to export its HTTP stats. We also used net/http/pprof's behavior of registering to the default global HTTP ServeMux, via DebugServer, which starts an HTTP server that uses that global ServeMux.
In this change, I merge DebugServer's functions into StatsAndLogging. StatsAndLogging now takes an address parameter and fires off an HTTP server in a goroutine. That HTTP server is newly defined, and doesn't use DefaultServeMux. On it is registered the Prometheus stats handler, and handlers for the various pprof traces. In the process I split StatsAndLogging internally into two functions: makeStats and MakeLogger. I didn't port across the expvar variable exporting, which serves a similar function to Prometheus stats but which we never use.
One nice immediate effect of this change: Since StatsAndLogging now requires and address, I noticed a bunch of commands that called StatsAndLogging, and passed around the resulting Scope, but never made use of it because they didn't run a DebugServer. Under the old StatsD world, these command still could have exported their stats by pushing, but since we moved to Prometheus their stats stopped being collected. We haven't used any of these stats, so instead of adding debug ports to all short-lived commands, or setting up a push gateway, I simply removed them and switched those commands to initialize only a Logger, no stats.
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