Add function name to tracing

Currently the `Activity` started for an invocation is called `Invoke` and it contains the invocation id as tag `faas.execution`.

https://github.com/Azure/azure-functions-dotnet-worker/blob/main/src/DotNetWorker.Core/Diagnostics/FunctionActivitySourceFactory.cs#L63

However, I find the function name in such context more useful than the invocation id.

Is it possible to add the function name as tag to that activity?

Start by reading src/DotNetWorker.Core/Diagnostics/FunctionActivitySourceFactory.cs around line 63, where the invocation Activity is created. Trace the invocation data available there and determine how the function name should be exposed as an Activity tag. Done means the tracing activity includes the function name while retaining the existing invocation identifier tag.

Written by the indexing model from the issue text.