Add "alerts" to metrics.

[rsyi]

We want to add alerting support within metrics yaml definitions, so given a user-provided condition, a slack message will be sent.

Detailed requirements

Metrics can be defined that assess the quality of data in a table, e.g. by using a definition like:

id-nulls:
  sql: |
    select count(id) from schema.table where id is not null

This would be substantially more useful if coupled with alert setup, which would tell you when a field is broken. Perhaps something like this would be reasonable:

id-nulls:
  sql: |
    select count(id) from schema.table where id is not null
  alerts:
    - condition: "> 0"
      message: "Nulls found in schema.table, column 'id'."
      slack: ["#channel", "@somebody"]
      email: "robert@github.com"

This would require support for several things:

• Scaffolding to send slack messages: Here is the official SDK, which provides a nice example on how to do this.
• Triggering the send of slack messages: Within MetricRunner, probably, loop through the alerts section of the yaml run eval on the checks against the result, and, given a failure, send a message to the slack recipients specified with the message message.

Future considerations:

• Email support would also be nice, though as a heavy slack user, I don't see a huge need for it, so will leave it out-of-scope until someone else argues. :)