Fixing resource auto-inference algorithm (#465)

Author: eyalkoren

specs/agents/tracing-spans-destination.md

@@ -100,11 +100,25 @@ exit = exit || context.destination || context.db || context.message || context.h
 
 If no value is set to the `context.destination.service.resource` field, the logic for automatically inferring 
 it MUST be the following:
+
 ```groovy
-if      (context.db?.instance)         "${subtype ?: type}/${context.db?.instance}"
-else if (context.message?.queue?.name) "${subtype ?: type}/${context.message.queue.name}"
-else if (context.http?.url)            "${context.http.url.host}:${context.http.url.port}"
-else                                   subtype ?: type
+if (context.db)
+  if (context.db.instance)
+    "${subtype ?: type}/${context.db.instance}"
+  else
+    subtype ?: type
+else if (context.message)
+  if (context.message.queue?.name) 
+    "${subtype ?: type}/${context.message.queue.name}"
+  else
+    subtype ?: type
+else if (context.http?.url)
+  if (context.http.url.port > 0)  
+    "${context.http.url.host}:${context.http.url.port}"
+  else if (context.http.url.host)
+    context.http.url.host
+else 
+  subtype ?: type
 ```
 
 If an agent API was used to set the `context.destination.service.resource` to `null` or an empty string, agents MUST