Support Azure VM Agent about nanos HOT 3 CLOSED

#2022 implements an Azure VM agent and a diagnostic extension that publishes memory metrics. Please note that the extension is enabled and configured via manifest options when creating a Nanos image; it sends metrics regardless of whether the diagnostic extension is enabled in the Azure portal, but in order for these metrics to show up in the charts you need to enable the extension (in fact, without enabling the extension you cannot even select guest OS metrics in the charts).

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Guest OS metrics such as memory usage are only available for a given VM or VM Scale Set if the Azure diagnostic extension has been enabled in that VM or VMSS. Otherwise, such metrics cannot be viewed in the Metrics section of the Azure portal, and cannot be used in autoscale trigger rules, even if the VM is correctly sending metrics data; one of the reasons for this is that metrics data is stored in tables in a given storage account, and the Azure portal needs to know what storage account is being used and takes this information from the configuration settings of the diagnostic extension.
Enabling an extension (either via a deployment template for a VMSS, or via the Azure portal or the Azure client for a VM) requires communication with the VM agent running in a VM, which is instructed to download, install, configure, and run the extension; if no agent is running, enabling the extension fails, and guest OS metrics are not available. So if we want this to work on Nanos, we have to implement (in addition to the code that sends the metrics to a storage account) an agent that responds to requests to enable extensions (and possibly to requests for configuration settings of a given extension).

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Awesome! Thanks!

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