monitoring: generate alerts as native prometheus alerts, let alert_count depend on alert rules

Created by: bobheadxi

related:

• out-of-band alerts (defined as prometheus alert rules) don't line up with the rest of our alerts in dashboards, etc: https://sourcegraph.slack.com/archives/CJX299FGE/p1595254363107300
• difficult to express certain alert types purely with recording rules (ie alert_count): https://github.com/sourcegraph/sourcegraph/issues/12158

potential solution: https://sourcegraph.slack.com/archives/CJX299FGE/p1594994195043000?thread_ts=1594971987.034300&cid=CJX299FGE

we could continue to support alert_count by inverting the dependency - have the alert rule make the query, make alert_count recording rules something like ALERTS{alertname="blah"} > 0

cc @keegancsmith