feat: send user agent as a separate attribute (#79)

## Which problem is this PR solving?

- Closes #76 

## Short description of the changes
- Pulls out `User-Agent` header and sets the `user_agent.original`
attribute

## How to verify that this has the expected result

<img width="606" alt="Screenshot 2023-08-16 at 11 49 55 AM"
src="https://github.com/honeycombio/honeycomb-ebpf-agent/assets/8810222/96032f66-e03a-4b04-8c4f-734e4717e08b">

main.go

@@ -102,21 +102,28 @@ func handleHttpEvents(events chan assemblers.HttpEvent, client *kubernetes.Clien
 		select {
 		case event := <-events:
 
+			// create libhoney event
 			ev := libhoney.NewEvent()
+
+			// common attributes
 			ev.AddField("duration_ms", event.Duration.Microseconds())
 			ev.AddField(string(semconv.NetSockHostAddrKey), event.SrcIp)
 			ev.AddField("destination.address", event.DstIp)
+
+			// request attributes
 			if event.Request != nil {
 				ev.AddField("name", fmt.Sprintf("HTTP %s", event.Request.Method))
 				ev.AddField(string(semconv.HTTPMethodKey), event.Request.Method)
 				ev.AddField(string(semconv.HTTPURLKey), event.Request.RequestURI)
 				ev.AddField("http.request.body", fmt.Sprintf("%v", event.Request.Body))
 				ev.AddField("http.request.headers", fmt.Sprintf("%v", event.Request.Header))
+				ev.AddField(string(semconv.UserAgentOriginalKey), event.Request.Header.Get("User-Agent"))
 			} else {
 				ev.AddField("name", "HTTP")
 				ev.AddField("http.request.missing", "no request on this event")
 			}
 
+			// response attributes
 			if event.Response != nil {
 				ev.AddField(string(semconv.HTTPStatusCodeKey), event.Response.StatusCode)
 				ev.AddField("http.response.body", event.Response.Body)
@@ -125,7 +132,7 @@ func handleHttpEvents(events chan assemblers.HttpEvent, client *kubernetes.Clien
 				ev.AddField("http.response.missing", "no response on this event")
 			}
 
-			// k8s metadata
+			// k8s attributes
 			k8sEventAttrs := utils.GetK8sEventAttrs(client, event.SrcIp, event.DstIp)
 			ev.Add(k8sEventAttrs)