fix(metrics): Support multiple addMetric() call with the same metric name

[ijemmy]

Description of your changes

Currently, adding metrics with the same name will override the existing value. For example,

metrics.addMetric('successfulBooking', MetricUnits.Count, 2);
metrics.addMetric('successfulBooking', MetricUnits.Count, 1);

Will emit only one EMF metric with value "1" (the 2nd call override the first one).

This is not the correct behavior. This PR implements the change to emit two values in array like this:

    {
        "performedActionA": [ // Use array instead of number when there're multiple values
            2,
            1
        ],
        "_aws": {
            "Timestamp": 1592234975665,
            "CloudWatchMetrics": [
                {
                "Namespace": "serverlessAirline",
                //...
                "Metrics": [
                    {
                    "Name": "performedActionA",
                    "Unit": "Count"
                    }
                ]
                }
            ]
        },
        //...
    }

How to verify this change

Run test for metrics package. I have added test cases in Metrics.test.ts and refactor the test for middy to reflect this change

Related issues, RFCs

#384

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

Breaking change checklist

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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[ijemmy]

Not practical to run e2e test in the watch mode so I filtered this.

[flochaz]

good point !

[ijemmy]

Just shifting indentation. The current version misses one more indentation under "_aws"

[ijemmy]

@heitorlessa Ping, in case you want to port this to other languages.

[heitorlessa]

Suggestion to improve error thrown and docs section name

[flochaz]

good point !

[saragerion]

Fantastic!

[saragerion]

Nice 🚀🚀🚀