The GetHeader method returns the value of a response header
from the last request made to the service provider.
This method is intended for applications that need lower-level access to
provider-specific response headers. Most applications do not need to call
this method. Standard response information, such as the HTTP status code,
response text, token usage, model name, and elapsed time, is available
through the control's properties.
The headers returned by a service provider can vary depending on the
provider, model, endpoint, account configuration, and the specific request
being made. Applications should not assume that a particular header will
always be present unless it is documented by the service provider.
If no response headers are available, or if the specified header was not
returned by the server, the HeaderValue parameter will contain
an empty string when the method returns.