Emit warning if APPINSIGHTS_INSTRUMENTATIONKEY isn't set, but Distributed Tracing is enabled

[bachuv]

Currently, if a customer enables distributed tracing in the app's host.json, but doesn't set APPINSIGHTS_INSTRUMENTATIONKEY as an environment variable then startup will fail saying that APPINSIGHTS_INSTRUMENTATIONKEY isn't set.

This PR allows startup to succeed if Distributed Tracing is enabled, but APPINSIGHTS_INSTRUMENTATIONKEY isn't set. If APPINSIGHTS_INSTRUMENTATIONKEY isn't set then the extension will emit this warning: 'APPINSIGHTS_INSTRUMENTATIONKEY' isn't defined in the current environment variables, but Distributed Tracing is enabled. Please set 'APPINSIGHTS_INSTRUMENTATIONKEY' to use Distributed Tracing..

Resolves #1694

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation PR is ready to merge and referenced in pending_docs.md
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[ConnorMcMahon]

A couple of small changes, but otherwise looks good!

[ConnorMcMahon]

LGTM!