Support JSON formatting of logs

[carlpett]

common/log supports a json formatter for the log output, which common/promlog does not. Would a PR to add a flag for switching between json/plain text be acceptable? This would make it much easier to integrate exporters etc into environments which mandate structured logging (vs having an extra binary to rewrite the logs).
go-kit already has the necessary NewJSONLogger function, which could be conditionally used instead of NewLogfmtLogger into here:

common/promlog/log.go

Lines 58 to 63 in c7de230

	func New(al AllowedLevel) log.Logger {
	l := log.NewLogfmtLogger(log.NewSyncWriter(os.Stderr))
	l = level.NewFilter(l, al.o)
	l = log.With(l, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)
	return l
	}

[brian-brazil]

See #136

[carlpett]

Ah, I just searched issues, not PRs :) Great!

[simonpasquier]

Closing as #136 is now merged.