Add custom query telemetry and response header to OData endpoint

[joelverhagen]

Progress on https://github.com/NuGet/Engineering/issues/1798.

This PR does two things:

1. Add custom metric ODataCustomQuery to OData queries.
   • This has a property IsCustomQuery = True | False | Unknown
2. Add a response header X-NuGet-CustomQuery: true | false
   • This can be used by APIM

The telemetry will allow us to verify that official clients are not doing custom queries. Note that "custom query" does not necessarily mean executed without the database. I chose to make this distinction for the GetUpdates OData function. This can be called by official client (2.x for many cases, 3.x+ for nuget.exe update -self) so it should not be throttled like custom OData queries.

Another case: when search hijack throws an exception, some (maybe all?) codepaths fall back to DB. This should probably not be throttled...

[joelverhagen]

Another case: when search hijack throws an exception, some (maybe all?) codepaths fall back to DB. This should probably not be throttled...

I've solved this case with an "Unknown" state. This is registered as "Unknown" in telemetry and no X-NuGet-CustomQuery header in the response body.

[loic-sharma]

👍 Nice test additions

[joelverhagen]

Tried this out on DEV environment.

<rate-limit-by-key
    calls="10"
    renewal-period="60"
    counter-key="@(context.Request.IpAddress)"
    increment-condition="@(context.Response.Headers.GetValueOrDefault("X-NuGet-CustomQuery", "false") == "true")" />

Works great!

[joelverhagen]

I would like to ship this to PROD without the throttle rule and observe the telemetry. Any objections?