Implement dimension settings using MetricDeclarations

[kohrapha]

Description
This PR implements the necessary functionality to perform dimension setting for exported metrics, as required by the ECS team. Specifically, this PR accomplishes the following:

1. Create a new metric_declarations attribute in the config file which specifies the rules used for dimension setting.
2. Apply rules to metrics whose metric names match at least one of the specified metric_name_selectors for that rule. If no selectors are defined, this rule is dropped as invalid.
3. Set exported dimensions (within _aws field) using the dimensions specified by the metric declaration rules. If the metric does not match any rules or specified dimensions, drop the metric (i.e. don't include the _aws field). If any specified dimensions exceed 10 elements, drop them as CW does not support them.
4. Ensure that dimension rollup works well with dimension setting.
5. Implement tests for the new functionality.

Additionally, the following sanity checks for user-defined metric declarations are implemented:

1. Duplicated dimensions in a dimension set are removed (i.e. ['dim1', 'dim2', 'dim1'] -> ['dim1', 'dim2'])
2. Duplicated dimension sets in a metric declaration are removed (i.e. [['dim1', 'dim2'], ['dim2', 'dim1']] -> [['dim1', 'dim2']])
3. Dimension sets with more than 10 dimensions are removed.

In all 3 cases, we output a warning message informing the user of the dropped elements. More information about this dedup logic can be found in #6.

Testing
This code change has been tested with unit tests and in a sample environment using an NGINX server on EKS. Given the following config:

exporters:
  awsemf:
    log_group_name: 'awscollector-test'
    region: 'us-west-2'
    log_stream_name: metric-declarations
    dimension_rollup_option: 'NoDimensionRollup'
    metric_declarations:
    - dimensions: [['Service', 'Namespace'], ['pod_name', 'container_name']]
      metric_name_selectors:
      - '^go_memstats_alloc_bytes_total$'
    - dimensions: [['app_kubernetes_io_component', 'Namespace'], ['app_kubernetes_io_name'], ['Invalid', 'Namespace']]
      metric_name_selectors:
      - '^go_goroutines$'
    - dimensions: [['Namespace', 'Service', 'Namespace']]
      metric_name_selectors:
      - '^go_mem.+$'

The exported CW logs look like this:

1 matching rule (w/ 1 unmatched dimension):
go_goroutines:

{
    "Namespace": "eks-aoc",
    "Service": "my-nginx-ingress-nginx-controller-metrics",
    "_aws": {
        "CloudWatchMetrics": [
            {
                "Namespace": "metric-declarations",
                "Dimensions": [
                    [
                        "Namespace",
                        "app_kubernetes_io_component"
                    ],
                    [
                        "app_kubernetes_io_name"
                    ]
                ],
                "Metrics": [
                    {
                        "Name": "go_goroutines",
                        "Unit": ""
                    }
                ]
            }
        ],
        "Timestamp": 1604431600752
    },
    "app_kubernetes_io_component": "controller",
    "app_kubernetes_io_instance": "my-nginx",
    "app_kubernetes_io_managed_by": "Helm",
    "app_kubernetes_io_name": "ingress-nginx",
    "app_kubernetes_io_version": "0.40.2",
    "container_name": "controller",
    "go_goroutines": 93,
    "helm_sh_chart": "ingress-nginx-3.7.1",
    "kubernetes_node": "ip-192-168-69-5.us-west-2.compute.internal",
    "pod_name": "my-nginx-ingress-nginx-controller-77d5fd6977-mlc8k"
}

1 matching rule (w/ dedup):
go_memstats_alloc_bytes:

{
    "Namespace": "eks-aoc",
    "Service": "my-nginx-ingress-nginx-controller-metrics",
    "_aws": {
        "CloudWatchMetrics": [
            {
                "Namespace": "metric-declarations",
                "Dimensions": [
                    [
                        "Namespace",
                        "Service"
                    ]
                ],
                "Metrics": [
                    {
                        "Name": "go_memstats_alloc_bytes",
                        "Unit": "By"
                    }
                ]
            }
        ],
        "Timestamp": 1604431600752
    },
    "app_kubernetes_io_component": "controller",
    "app_kubernetes_io_instance": "my-nginx",
    "app_kubernetes_io_managed_by": "Helm",
    "app_kubernetes_io_name": "ingress-nginx",
    "app_kubernetes_io_version": "0.40.2",
    "container_name": "controller",
    "go_memstats_alloc_bytes": 6938768,
    "helm_sh_chart": "ingress-nginx-3.7.1",
    "kubernetes_node": "ip-192-168-69-5.us-west-2.compute.internal",
    "pod_name": "my-nginx-ingress-nginx-controller-77d5fd6977-mlc8k"
}

2 matching rules w/ dedup):

{
    "Namespace": "eks-aoc",
    "Service": "my-nginx-ingress-nginx-controller-metrics",
    "_aws": {
        "CloudWatchMetrics": [
            {
                "Namespace": "metric-declarations",
                "Dimensions": [
                    [
                        "Namespace",
                        "Service"
                    ],
                    [
                        "container_name",
                        "pod_name"
                    ]
                ],
                "Metrics": [
                    {
                        "Name": "go_memstats_alloc_bytes_total",
                        "Unit": ""
                    }
                ]
            }
        ],
        "Timestamp": 1604431600752
    },
    "app_kubernetes_io_component": "controller",
    "app_kubernetes_io_instance": "my-nginx",
    "app_kubernetes_io_managed_by": "Helm",
    "app_kubernetes_io_name": "ingress-nginx",
    "app_kubernetes_io_version": "0.40.2",
    "container_name": "controller",
    "go_memstats_alloc_bytes_total": 0,
    "helm_sh_chart": "ingress-nginx-3.7.1",
    "kubernetes_node": "ip-192-168-69-5.us-west-2.compute.internal",
    "pod_name": "my-nginx-ingress-nginx-controller-77d5fd6977-mlc8k"
}

No matching rule:

{
    "Namespace": "eks-aoc",
    "Service": "my-nginx-ingress-nginx-controller-metrics",
    "app_kubernetes_io_component": "controller",
    "app_kubernetes_io_instance": "my-nginx",
    "app_kubernetes_io_managed_by": "Helm",
    "app_kubernetes_io_name": "ingress-nginx",
    "app_kubernetes_io_version": "0.40.2",
    "container_name": "controller",
    "go_info": 1,
    "helm_sh_chart": "ingress-nginx-3.7.1",
    "kubernetes_node": "ip-192-168-69-5.us-west-2.compute.internal",
    "pod_name": "my-nginx-ingress-nginx-controller-77d5fd6977-mlc8k",
    "version": "go1.15.2"
}

Documentation
The documentation in the README for the awsemfexporter is also updated to include this new API change.

[kohrapha]

Tagging @mxiamxia

[kohrapha]

Opened PR to upstream: open-telemetry#1503