set up google cloud monitoring for alerts

[snarfed]

it's kind of embarrassing that we've gone this long without alerting. :P the ereporter setup (#211) is nice, but only for uncaught exceptions, and only sends a daily report.

i've turned on google cloud monitoring, which has basic app engine monitoring metrics built in. here's the dashboard.

we'd probably only alert on http response codes, latency, and instance count. i'm not planning a pager rotation or expecting to wake up in the middle of the night, but emails and maybe smses or push notifications will be nice.

funny timing: i'm setting up basic monitoring and alerting for huffduff-video on amazon cloudwatch right now too: snarfed/huffduff-video#7. comparing and contrasting will be interesting.

[snarfed]

done: https://app.google.stackdriver.com/policy-advanced

i set up these alerts:

1. Active instances over 5 for >15m
2. 4XX responses over 0.1 qps or 5xx responses over .05 qps for >15m

they email me when they trigger. if anyone else wants to get emails too (or SMSes or Slack messages or whatever), just ping me.