Support local development and execution of Azure Functions when using the Core Tools

[stevejgordon]

Currently, when the ApmMiddleware from Elastic.Apm.Azure.Functions is registered, traces will fail at APM server, if running the code using the Core Tools locally. This occurs because we expect to read various values used to populate the metadata from environment variables and these are not present in a developer environment.

A workaround is to include the following variables in the local.settings.json file to simulate those we require.

"FUNCTIONS_EXTENSION_VERSION": "Local",
"WEBSITE_OWNER_NAME": "SUB+RESOURCEGROUP-REGION",
"WEBSITE_SITE_NAME": "Local",
"REGION_NAME": "Region",
"WEBSITE_RESOURCE_GROUP": "Resource Group"

We should investigate introducing a fallback for simulating the metadata service runtime name when running locally. Ideally, we would find a heuristic to detect this scenario.