Plugins: Add AWS/MediaLive metric for CloudWatch

[arabian9ts]

What is this feature?

This PR adds AWS/MediaLive metric for CloudWatch plugin.
I extracted metric names and dimensions using the commands below.

metric names
$ aws cloudwatch list-metrics --namespace AWS/MediaLive --region ap-northeast-1 | jq '.Metrics[].MetricName' | sort | uniq

dimensions
$ aws cloudwatch list-metrics --namespace AWS/MediaLive --region ap-northeast-1 | jq '.Metrics[].Dimensions[].Name' | sort | uniq

cf. https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/viewing_metrics_with_cloudwatch.html

Why do we need this feature?

I'm building grafana dashboard for the video streaming service.

Who is this feature for?

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[CLAassistant]

All committers have signed the CLA.

[fridgepoet]

Thanks for your contribution!