[Metricbeat] Adding AWS Network ELB Metricset

[kaiyan-sheng]

A list of AWS Network ELB metrics from Cloudwatch:
https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-cloudwatch-metrics.html

Metricbeat Module / Dataset release checklist

This checklist is intended for Devs which create or update a module to make sure modules are consistent.

Modules

For a metricset to go GA, the following criterias should be met:

• Integration tests exist
• System tests exist
• Automated checks that all fields are documented
• Documentation
• Fields follow ECS and naming conventions
• Dashboards exists (if applicable)

Metricbeat module

• Example data.json exists and an automated way to generate it exists (go test -data)

[kaiyan-sheng]

Implemented as a part of #14123