Try out success rate metric on processing CSRs

Summary:
This is semi-explorative to see how far we can get with this approach, but I think it'l be useful. If it is, we can factor the code into a nicer utility, that perhaps wraps promises or even just has succeeded/failed methods.

Using 1 and 0 instead of true and false so we can do numerical aggregates on it, like average, which will give the success rate.

This will allow queries such as:
* What percentage of attempts fail? - Not very useful because they get retried if they fail.
* What percentage of sessions that attempt, encounter a failure? - More useful, but still not perfect because they could retry and succeed.
* What percentage of sessions attempt and don't ever succeed? - Better but still not perfect because one device might succeed and another fail, but this will be rare.

Reviewed By: passy

Differential Revision: D13625987

fbshipit-source-id: 2b9f45cc888247d068cc52281443e9ece7985da8

src/server.js

@@ -190,10 +190,20 @@ export default class Server extends EventEmitter {
                   client,
                   deviceId: result.deviceId,
                 });
+                this.logger.track(
+                  'success-rate',
+                  'processCertificateSigningRequest',
+                  1,
+                );
               })
               .catch(e => {
                 subscriber.onError(e);
                 this.emit('client-setup-error', {client, error: e});
+                this.logger.track(
+                  'success-rate',
+                  'processCertificateSigningRequest',
+                  0,
+                );
               });
           });
         }