[feature-request] Provide an option to set http client options, like headers or maybe update the http req.

I would like to pass contextual data to the Getter interface Get function via ctx.

For example, using tracing tools like Datadog, the client HTTP headers need to be added before the HTTP request is sent and pulled off when the loading function is run. That could allow complete application visibility:
https://docs.datadoghq.com/tracing/setup_overview/custom_instrumentation/go/#distributed-tracing

It could also allow the caller to pass other context-related information into the fill function. It seems like receiving side is possible by providing a custom Context func:
https://github.com/mailgun/groupcache/blob/master/http.go#L74

However, it does not look like there is any way currently to add onto the HTTP client request:
https://github.com/mailgun/groupcache/blob/master/http.go#L244

Start with http.go around the custom Context function at line 74 and the HTTP request at line 244. Trace how the Getter interface's Get function and fill function receive context, then determine how callers could provide request data such as tracing headers before the HTTP request is sent. Done means the requested contextual information can reach both sides without breaking existing usage.