metrics-collector - split D2C messages greater than limit

[LiamDavey]

We currently use metrics-collector configured with environment variable IoTMessage to send metrics as D2C messages upstream to IoT Hub. The current version of metrics-collector does not split D2C messages, even if they are greater than the D2C message limit (256KB). This results in us currently getting a MessageTooLargeException when trying to scrape our custom modules in addition to edgeHub and edgeAgent. This PR adds functionality to split D2C messages before sending upstream if messages are greater than limit.

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• Pull request includes test coverage for the included changes.
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  • concise summary of tests added/modified
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[LiamDavey]

@microsoft-github-policy-service agree

[vasdee]

Any chance this could be reviewed and merged? It's a very handy feature and we are currently having to run patched versions of this in order to work around the issue