[Metricbeat] Add collecting Application ELB and Network ELB into elb metricset

[kaiyan-sheng]

This PR is add support for application ELB and network ELB both into elb metricset. Now elb metricset should collect all cloudwatch metrics from classic ELB, application ELB and network ELB.

With this implementation, when elb metricset is enabled, metrics from all three different kinds of ELBs will be collected. This way is more consistent with Filebeat elb fileset.

closes #13538 and #13539

[jsoriano]

LGTM, could we have some example data.json?

[kaiyan-sheng]

@jsoriano Will do! And update the docs.

[kaiyan-sheng]

@jsoriano I regenerated the data.json https://github.com/elastic/beats/pull/14123/files#diff-30cbed3a1b919647b6d2d2ad59e5292b but since there are three different namespaces used in this metricset, we will get three separate metrics.
Are we expecting only one metric here?

[jsoriano]

@jsoriano I regenerated the data.json https://github.com/elastic/beats/pull/14123/files#diff-30cbed3a1b919647b6d2d2ad59e5292b but since there are three different namespaces used in this metricset, we will get three separate metrics.
Are we expecting only one metric here?

It is possible to select different events to be written in different files from TestData with WriteEventsCond (instead of WriteEvents). I don't think we are using it yet in other places, but with the old API there are some examples like this one: https://github.com/elastic/beats/blob/v7.4.2/metricbeat/module/system/socket/socket_test.go#L72