Add telemetry to the custom metadata provider path

[liliankasem]

Related to: Azure/azure-functions-dotnet-worker#788

When the host is reading metadata via a custom metadata provider (code), we do not validate the metadata like we do in our other code path (i.e. the worker metadata provider).

Without the validation, function names can have unwanted characters such as / which can lead to further failures down the line i.e. with function keys, storage etc.

Updating the custom provider code path to validate the metadata would be a breaking change. To analyze if this should be a hard failure or if we should make any further changes here, we should add telemetry to this code path to give us a good idea of how many Function Apps would be impacted if we made changes here.