Change aws_elb autodiscover provider meta field prefix to aws.elb

[kaiyan-sheng]

This issue came from comment #14823 (comment) when adding aws_ec2 autodiscover provider in #14823. In aws_ec2 provider, meta fields are started with aws.ec2 to match metrics from aws module Metricbeat.

Similarly, it would be good to update aws_elb provider to also have aws.elb prefix for meta fields. Also meta fields are not added to autodiscover template, similar to #14823 (comment).

[kaiyan-sheng]

@andrewvc aws_elb is experimental, does that mean we can make the change without waiting till 8.0.0?

[andrewvc]

That's a good point @kaiyan-sheng , given the experimental status I'm OK with this. Do you mind updating the docs as well?

[kaiyan-sheng]

@andrewvc Will do, thanks!