Activity tag propogation discussion

[andystaples]

Discussion:
Currently, we propagate all custom tags added to Activity.Current to all child spans on creation. The intent behind this decision was to ensure that all user spans contained the InvocationId as a property. However, this means that if a user defines a span, adds a tag, and then creates a child span, it will inherit these tags as well, which is divergent behavior from the System.Diagnostics Activity class that our spans are based on. Is this an issue, should we limit propagation to InvocationId? Limit it further to InvocationId to spans that are direct children of the internal span? or remove it entirely? Open to discussion