FastBoot should emit metrics

[ember-fastboot]

FastBoot should emit metrics

[sdhull]

notes from discussion in fastboot meeting:

• would tooling be low-overhead and on all the time?
• would there be some debugging mode that you can turn on just to debug?
• are there specific metrics you want?
• time from when the visit request started to when the promise was resolved
• when rehydration starts, how long does it take?
• we're not going to do better than existing node.js tools re: memory leaks & stuff
• express & node have tooling
• what kind of unique-to-ember things can we emit?
• did a rerender get scheduled?
• did you use a tag that is known to have issues with rehydration? (eg <select> [aside: wait, for real?])

@ryanto has some middlewares for some tooling he might be willing to share...?

[rondale-sc]

I believe in the rehydration builder there is a path that happens when a node is skipped or removed (when the rehydration builder can't decipher something so it throws it out). It would be great to see a counter of that accessible in some sort of debug mode to determine how effective rehydration is.