Fix timestamps when metrics are submitted synchronously with date

[sfirrin]

What does this PR do?

Fixes a bug where the timestamp set with synchronous metrics was submitted to Datadog with the timestamp in milliseconds rather than the seconds that the API requires.

Metrics submitted with a timestamp in the future (i.e. a millisecond timestamp that's interpreted as a second timestamp) are given the current time, and this is handled in the async metric path, so this bug was only affecting sync metrics submitted with a date in the past, functionality added in #63.

Motivation

#67

Testing Guidelines

Tested this and it's working as intended.

Old log (millisecond timestamps):

{
    "status": "debug",
    "message": "datadog:sending payload with body {\"series\":[{\"metric\":\"lambda.test.dist_past\",\"points\":[[1585856360814,[123]]],\"tags\":[\"dd_lambda_layer:datadog-nodev12.16.1\"],\"type\":\"distribution\"},{\"metric\":\"lambda.test.dist_now\",\"points\":[[1585856364414,[123]]],\"tags\":[\"dd_lambda_layer:datadog-nodev12.16.1\"],\"type\":\"distribution\"}]}"
}

New log (second timestamps):

{
    "status": "debug",
    "message": "datadog:sending payload with body {\"series\":[{\"metric\":\"lambda.test.dist_past\",\"points\":[[1585939463,[123]]],\"tags\":[\"dd_lambda_layer:datadog-nodev12.16.1\"],\"type\":\"distribution\"},{\"metric\":\"lambda.test.dist_now\",\"points\":[[1585939467,[123]]],\"tags\":[\"dd_lambda_layer:datadog-nodev12.16.1\"],\"type\":\"distribution\"}]}"
}

[sfirrin]

I converted these second timestamps to milliseconds since Date's constructor takes milliseconds. The timestamps in the payloads being tested remain in seconds.