[Metricbeat] add collecting tags for rds metricset

[kaiyan-sheng]

What does this PR do?

This PR adds tag collection in rds metricset by default.

Why is it important?

Tags are important part of metadata for RDS instances. With this PR, users can collect metrics from RDS and use tags for filtering.

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

How to test this PR locally

Create RDS with tags first, then use config below to start aws module:

- module: aws
  period: 5m
  credential_profile_name: elastic-beats
  metricsets:
    - rds

You should see tags collected in events, for example:

{
    "@timestamp": "2017-10-12T08:05:34.853Z",
    "aws": {
        "rds": {
            "aurora_bin_log_replica_lag": 0,
            "aurora_replica.lag_max.ms": 19.108999252319336,
            "aurora_replica.lag_min.ms": 19.108999252319336,
            "cache_hit_ratio.buffer": 100,
            "cache_hit_ratio.result_set": 0,
            "cpu": {
                "total": {
                    "pct": 0.04
                }
            },
            "database_connections": 0,
            "db_instance": {
                "arn": "arn:aws:rds:us-east-1:428152502467:db:database-1-instance-1",
                "class": "db.r5.large",
                "identifier": "database-1-instance-1",
                "status": "available"
            },
            "db_instance.identifier": "database-1-instance-1",
            "deadlocks": 0,
            "disk_usage": {
                "bin_log.bytes": 0
            },
            "engine_uptime.sec": 2704277,
            "free_local_storage.bytes": 31745863680,
            "freeable_memory.bytes": 4634234880,
            "latency": {
                "commit": 5.270933333333333,
                "ddl": 0,
                "delete": 0,
                "dml": 0.1624,
                "insert": 0.1624,
                "select": 0.17333862433862435,
                "update": 0
            },
            "login_failures": 0,
            "queries": 9.11833836203304,
            "throughput": {
                "commit": 0.5000916834753039,
                "ddl": 0,
                "delete": 0,
                "dml": 0.5000916834753039,
                "insert": 0.5000916834753039,
                "network": 1.4,
                "network_receive": 0.7,
                "network_transmit": 0.7,
                "select": 3.150577605894414,
                "update": 0
            },
            "transactions": {
                "active": 0,
                "blocked": 0
            }
        },
        "tags": {
            "created-by": "ks",
            "dept": "engr"
        }
    },
    "cloud": {
        "account": {
            "id": "428152502467",
            "name": "elastic-beats"
        },
        "availability_zone": "us-east-1b",
        "provider": "aws",
        "region": "us-east-1"
    },
    "event": {
        "dataset": "aws.rds",
        "duration": 115000,
        "module": "aws"
    },
    "metricset": {
        "name": "rds",
        "period": 10000
    },
    "service": {
        "type": "aws"
    }
}

Also test with tags_filter:

- module: aws
  period: 5m
  credential_profile_name: elastic-beats
  tags_filter:
      - key: "dept"
        value: "eng"
  metricsets:
    - rds

Related issues

• Closes Add support to collects tags in RDS (AWS module) #16358

[ChrsMark]

Overall it looks good to me. I left just one small comment :)