Improve cert-checker's log and metric output

[aarongable]

Currently cert-checker produces its output by printing out a multi-line fully-indented huge json blob:

boulder/cmd/cert-checker/main.go

Lines 71 to 78 in c62e796

	func (r *report) dump() error {
	content, err := json.MarshalIndent(r, "", " ")
	if err != nil {
	return err
	}
	fmt.Fprintln(os.Stdout, string(content))
	return nil
	}

It prints this directly to stdout, bypassing the configured logger (probably because syslog would mangle the multi-line json). Also, it doesn't collect any prometheus metrics, because it's a shortlived process that might not be scraped during its runtime.

But internally, we have a python wrapper that:

• Runs cert-checker and captures its stdout
• Parses the JSON and re-emits it in a different format to a configured logger
• Produces prometheus metrics based on the count of good and bad certificates found
• Pushes those metrics to a prometheus endpoint, which it discovers via consul service lookup

We can do all of those things directly in Boulder! While the python script may have been helpful in a time before consul service discovery, it doesn't seem like it serves any critical purpose anymore. Let's improve cert-checker's audit log and metric output, so the wrapper is no longer necessary.