Fix parsing metrics with decimal timestamp

[nhulston]

What does this PR do?

When receiving metrics over dogstatsd, we receive metrics in the format: metric.name:15|c|#env:dev|T1656581409, where T1656581409 is the timestamp.

We parse the timestamp as i64, which means that if the timestamp is a float such as T1656581409.123, the parse will fail, and we fallback to now.

Motivation

Just for flexibility. Even though metrics are rounded to the nearest 10 second bucket, we should still support decimal input.

E.g. v108 of datadog-lambda-python sends timestamped metrics with decimals. Therefore, we currently fallback to now since the parse as u64 fails.

Additional Notes

Describe how to test/QA your changes

I tested manually with local builds of Node+Extension, where Node was sending timestamps with a decimal:

• Used datadog-lambda-js branch from Implement dogstatsd, add timestamp support, fix flushing datadog-lambda-js#648
• Built my own lambda extension layer: arn:aws:lambda:us-east-1:<sandbox account>:layer:nhulston-datadog-extension:336

And timestamped metrics now work as expected:

Also added a basic unit test