We need to be able to modify the HTTP POST in Weave Cortex to add
multitenancy information to a notification. Since we only really need a
special header in the end, the other option would be to just allow
passing in headers to the notifier. But swapping out the whole Doer is
more general and allows others to swap out the network-talky bits of the
notifier for their own use. Doing this via contexts here wouldn't work
well, due to the decoupled flow of data in the notifier.
There was no existing interface containing the ctxhttp.Post() or
ctxhttp.Do() methods, so I settled on just using Do() as a swappable
function directly (and with a more minimal signature than Post).
The current description does not accurately describe when the metric is incremented.
Aside from Alertmanger missing from the configuration, `prometheus_notifications_dropped_total` is incremented when errors occur while sending alert notifications to Alertmanager, or because the notifications queue is full, or because the number of notifications to be sent exceeds the queue capacity.
I think calling these cases 'errors' in a generic sense is more useful than the current description.
This commit extends the notifier to dispatch alert batches
to multiple Alertmanagers concurrently.
It changes the `-alertmanager.url` flag to accept a comma
separated list of URLs and/or to be set multiple times.
