Fixes an issue introduce in #21288
The default sdkclient created by the CLI root includes several additional http.RoundTripper wrappers to check versions and attach telemetry, so `DupClientCopyingHeaders` would break and scale tests would fail.
Instead of explicitly adding support for these additional wrappers to `DupClientCopyingHeaders` I think we should just stop unwrapping and move on. Scale tests don't need these wrapped functions.
This is a bit fragile, since it depends on the fact that the headers wrapper needs to be outermost, but that needs to be true for other uses, since things like dialing DERP do a similar thing where they unwrap and extract the auth headers. More long term this needs a refactor to make HTTP headers in the SDK a more first-class resource instead of this hacky RoundTripper wrapping, but that's for a different day.
While scale testing, I noticed that our load generators send basically
all requests to a single Coderd instance.
e.g.
This is because our scale test commands create all `Runner`s using the
same codersdk Client, which means they share an underlying HTTP client.
With HTTP/2 a single TCP session can multiplex many different HTTP
requests (including websockets). So, it creates a single TCP connection
to a single coderd, and then sends all the requests down the one TCP
connections.
This PR modifies the `exp scaletest` load generator commands to create
an independent HTTP client per `Runner`. This means that each runner
will create its own TCP connection. This should help spread the load and
make a more realistic test, because in a real deployment, scaled out
load will be coming over different TCP connections.
Spike Curtis
