This should resolve the following exception for all SmokeTests except for SecurityGrantFileNotFoundExceptionSmokeTest: "System.TimeoutException : The smoke test is running for too long or was lost."

- Make MemberResult a struct
- Add a custom key type for the cache to avoid building a string every time
- Use the custom key to prevent closure allocation in the Concurrent.GetOrAdd callbacks
- Make the PropertyFetcher generic to prevent boxing when fetching a value type
- Add a Fetch overload to provide the type when it's already known
- Use expressions to fetch the property value

- Use a cached empty array
- Add generic overloads and eagerly fetch type to reduce boxing
- Remove boxing in key comparer
- Remove string allocations from key
- Embed the MethodBuilder in the key
- Add a WithParameters overload with 4 parameters

It looks like creating a DateTimeOffset because of the number of internal checks. We only need the Timespan in that codepath.

* Add diagnostic logs at startup
* Write the diagnostic message the first time the Tracer ctor is called
* Add an API to check the status of the agent and call it in the startup log

* Suppress heap allocations in TracerSettings.IsIntegrationEnabled
* Cache value in a static field + add unit test

…n IIS applications (#775)

Modify the profiler so that, when running IIS or IIS Express on .NET Framework, the startup hook is only inserted in the `System.Web.Compilation.BuildManager.InvokePreStartInitMethods`. This has the following effects:

1. This removes the guess-and-check nature of valid callsites for the startup logic when running IIS. Before, `System.Web.Compilation.BuildManager.InvokePreStartInitMethods` was called out as a valid callsite, but it was possible for other modules to be loaded and methods to be called before this, causing the startup logic to run in the incorrect place.
2. This leaves ASP.NET Core apps hosted on IIS and IIS Express unchanged as they will still load at the earliest possible time in the setup of the ASP.NET Core application.
3. For ASP.NET (.NET Framework) applications, additional AppDomains that are not the main AppDomain won't be automatically instrumented. This trade-off should be acceptable as any automatic tracing there would be disconnected from the original request anyways since it is separated by the AppDomain boundary.

* Cache the tags extracted from connection strings
* Disable the cache if too many different connection strings are used

)

* Improvement  in ToBoolean StringExtensions
* Use Equals instead Compare

- Reuse the same workers for each burst
- Remove the initialization/warmup time from the "elapsed" computation
- Simplify synchronization by using Barrier

* Reduce heap allocations in AspNetCoreDiagnosticObserver

- Use a wrapper on top of IHeaderDictionary instead of duplicating the headers
- Make all methods that expect a IHeadersCollection generic, to avoid boxing the wrapper
- Cache the SamplingPriority values to avoid the costly enum conversion
- Remove the ToList when accessing the header values

Reduce the number of intermediate strings.

…d HTTP calls (#780)

This PR improves the `Datadog.Trace.ClrProfiler.ScopeFactory.CreateOutboundHttpScope` helper method, used by both the `WebRequest` integration and the `HttpMessageHandler` integration, in the following ways:
1. Removes several possible locations for a `NullReferenceException` to occur in the method
2. Adds a more robust check on the parent span so that the HTTP span de-duplication logic (e.g. automatic instrumentation from nested `HttpMessageHandler` objects) will only consider spans created by automatic instrumentation, not custom spans

* Double the execution time of smoke tests

* Adds support for 'x-datadog-origin' http header
- Enable Extractor to parse the header and store it in the span context.
- Enable Injector to inject the span context origin property to propagate the value.
- Sets the '_dd.origin' tag to a new span if the span context has the origin value and if is the root span of a trace.
- New constant values for the http header and the span tag.
- Unit tests for Extract, Inject and setting the tag in the root span.

* Changes based on the review
- Changing the SpanContext Origin property to internal.
- Moving the Origin set tag block to the DecorateRootSpan method.
- Removing the ToList() and the double enumeration from the SpanContextPropagator.

* Changes based on the review:
- Adds the Origin header logic to SpanContextPropagatorHelpers.
- Changed the unit tests for SpanContextPropagatorHelpers to consider the new origin header.
- Fixes the OriginHeader_InjectFromChildSpan test.

* Restore previous ctor to avoid a binary breaking change and introducing a new internal ctor for the origin header.