Cherry-pick #16402 to 7.x: [libbeat] Change aws_elb autodiscover provider field name to aws.elb.*

[kaiyan-sheng]

Cherry-pick of PR #16402 to 7.x branch. Original message:

What does this PR do?

This PR is to change aws_elb autodiscover provider fields from elb_listener.* to aws.elb.*. For example, the output event will look like below from unit test:

map[aws:{"elb":{"availability_zones":["strVal"],"created":"2020-02-18T13:58:30.569437-07:00","host":"fake.example.net","ip_address_type":"dualstack","listener_arn":"listen_arn","load_balancer_arn":"lb_arn","port":1234,"protocol":"HTTPS","scheme":"internet-facing","security_groups":["foo-security-group"],"ssl_policy":"strVal","state":{"code":"active","reason":"strVal"},"type":"application","vpc_id":"strVal"}} cloud:{"availability_zone":["strVal"],"provider":"aws","region":"strV"} host:fake.example.net id:listen_arn meta:{"aws":{"elb":{"availability_zones":["strVal"],"created":"2020-02-18T13:58:30.569437-07:00","host":"fake.example.net","ip_address_type":"dualstack","listener_arn":"listen_arn","load_balancer_arn":"lb_arn","port":1234,"protocol":"HTTPS","scheme":"internet-facing","security_groups":["foo-security-group"],"ssl_policy":"strVal","state":{"code":"active","reason":"strVal"},"type":"application","vpc_id":"strVal"}},"cloud":{"availability_zone":["strVal"],"provider":"aws","region":"strV"}} port:1234 provider:39021adf-6e56-4c6a-8a3c-b12bf9317c06 start:true]

meta represents what will end up as metadata in the output of modules that get launched. This PR also updated aws_elb provider to have all the meta fields into autodiscover template by adding it to the root of the event. Please see #14823 (comment) for more details.

Why is it important?

With this change, fields from aws_elb and aws_ec2 autodiscover providers will match.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Closes Change aws_elb autodiscover provider meta field prefix to aws.elb #16219