Use server span when determining status from code

Switches to `SpanStatusFromHTTPStatusCodeAndSpanKind` with a span kind
of `SpanKindServer` so that 4xx status codes don't get labelled as
errors. Since these codes represent client errors, they make it
difficult to differentiate bad server behavior from bad client behavior.

instrumentation/github.com/gorilla/mux/otelmux/mux.go

@@ -144,7 +144,7 @@ func (tw traceware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	defer putRRW(rrw)
 	tw.handler.ServeHTTP(rrw.writer, r2)
 	attrs := semconv.HTTPAttributesFromHTTPStatusCode(rrw.status)
-	spanStatus, spanMessage := semconv.SpanStatusFromHTTPStatusCode(rrw.status)
+	spanStatus, spanMessage := semconv.SpanStatusFromHTTPStatusCodeAndSpanKind(rrw.status, oteltrace.SpanKindServer)
 	span.SetAttributes(attrs...)
 	span.SetStatus(spanStatus, spanMessage)
 }