[Metricbeat] Add tags for aws ec2

[kaiyan-sheng]

This pr is to add tags into metrics from aws module for ec2.
For ec2, tags are already returned from DescribeInstances api call. With this change, tags will be a part of the metrics reported by ec2 metricset:

    "aws": {
        "ec2": {
            "cpu": {
                "total": {
                    "pct": 0.7000000000116418
                }
            },
            "instance": {
                "core": {
                    "count": 1
                },
                "image": {
                    "id": "ami-62d35c02"
                },
                "monitoring": {
                    "state": "enabled"
                },
            }
        },
        "tags": {
            "Name": "service-node",
            "aws:autoscaling:groupName": "service-asg"
        }
    },

How to test it?

Start an AWS EC2 instance and add tags for it. Then after this instance is in running state, start metricbeat aws module with ec2 metricset enabled. Then you should be able to see tags being collected and stored under aws.tags.*.

[kaiyan-sheng]

I might found some other useful apis to do this. Will put this PR under in progress till I find more information.

[kaiyan-sheng]

I will keep this PR open for a little bit because the code added into ec2, s3 and sqs can still be used. There is no need to all call function GetResourcesTags in utils.go from #12480 when tag information is already obtained. For example, for ec2 metricset, DescribeInstances response already includes tags.