Update functions host to emit per invocation details to metering service

We have been receiving CRIs which complain that the FunctionExecutionCount unit shown in portal for Linux ElasticPremium apps is not accurate. This is because today the platform publishes an aggregated count for the FunctionExecutionCount metric. As a repair item for these CRIs, we'll start publishing accurate execution counts from the host.

The change needs to go here - https://github.com/Azure/azure-functions-host/blob/c63da9b6baec5155fcda4a7a233a1c26ea41bee9/src/WebJobs.Script.WebHost/Diagnostics/LinuxAppServiceEventGenerator.cs#L64-L70

Instead of just publishing the timestamp, we need to also publish the execution stage for each function invocation.
We have already made the corresponding change on the platform side to start consuming the new changes from the functions host. We have to wait for the platform changes to be deployed everywhere before making the change on the host side.