[Nullable Context] Enable on Microsoft.Diagnostics.Monitoring.WebApi

[schmittjoseph]

Summary

This is the first PR in a series to enable nullable aware context throughout our code. To help with ease of review and minimize the chance of introducing regressions, I'll be organizing the PRs as follows:

• Each project will have nullable aware context enabled in its own PR.

  • These PRs will be large but only contain trivial, low/no-risk, changes. Namely they'll primarily contain the addition of the new nullable syntaxes (? and !) and static analysis attributes (e.g. [NotNullWhen(..)]).
  • Nullable context will be disabled around pieces of code that contain some level of risk to update. These sections of code will be addressed in follow up PRs so they can be properly reviewed. (e.g. #nullable disable ... #nullable restore)
  • If you see anything that you feel like could introduce a regression or is non-trivial in these PRs, leave a comment and I'll pull out those changes.

• All PRs in this effort will not contain any breaking changes.

Release Notes Entry