CloudWatch: Add ALB RequestCountPerTarget metric

fix cloudwatch query error over 24h

(cloudwatch) show error message when cloudwatch datasource can't add

This commit adds the new ALB metric `RequestCountPerTarget` which was
released in July 2017.

Missing dot in aws credentials path

As far as i know the path hasn't changed, but the dot is missing in one place.

makes it possible to have a second condition requering
at least X points of data.

Update rpm.md

for error "The GPG keys listed for the "grafana" repository are already installed but they are not correct for this package."

ref #9476

Kafka alerting

Prometheus client lib support gzip by itself. Which caused the
response to be double gzipped sometimes. We should use the Grafana
middle ware instead.

closes #9464

* added images to annotation docs

* added gifs and png to annotations, minor code formatting fix in alertlist

Bugfix: Always verify TLS unless explicitly told otherwise

initial draft of plugin dev document