Make host performance metrics accessible through App Insights

Users need to be able to access data on how close their function app instance is getting to hitting the consumption plan limits. This data can be provided by https://github.com/Azure/azure-webjobs-sdk-script/blob/dev/src/WebJobs.Script/Scale/HostPerformanceManager.cs but the process is highly inconvenient.

This issue tracks emitting host performance metrics to app insights, so that users can track there usage against these limits over time.

See https://github.com/Azure/Azure-Functions/issues/692 for a standalone NuGet for reading these metrics at runtime.

Start by reading src/WebJobs.Script/Scale/HostPerformanceManager.cs to understand the available host performance data, then trace where App Insights telemetry is emitted. Check the related standalone NuGet issue for context. Done means users can track consumption-plan limit usage over time through App Insights.

Written by the indexing model from the issue text.