Make DiagnosticContextCollector thread-safe

[nblumhardt]

Looking at this again, it would be too easy to violate the single-threadedness of DiagnosticContextCollector in the current design - there's no indication in the API of IDiagnosticContext that there are any constraints on how its called.

This PR introduces a simple mutex to make DiagnosticContextCollector thread-safe. There's a possibility this could be done lock-free, but that's probably fine to punt for a future iteration.

After the PR, the example in #9 (comment) will be correct/safe:

var collector = _diagnosticContext.BeginCollection();
var task = SomeMethodAsync();
collector.TryComplete(...);

[nblumhardt]

Dropping in one more commit that switches Add() for Set() and updates existing values rather than extending the list of collected properties.

[nblumhardt]

Still churning this feature a bit, and keen to get the changes integrated downstream while I have some time available. If everyone will forgive me the indiscretion, I'll merge now recklessly 😁 and push any changes from review feedback in a follow-up PR.