refactor: move request and response types into separate header

include/neuron/define.h

@@ -86,4 +86,7 @@ typedef enum {
     NEU_TAG_CACHE_TYPE_NEVER,
 } neu_tag_cache_type_e;
 
+// forward declaration for neu_adapter_t
+typedef struct neu_adapter neu_adapter_t;
+
 #endif

include/neuron/metrics.h

@@ -30,9 +30,6 @@ extern "C" {
 #include "type.h"
 #include "utils/uthash.h"
 
-// forward declaration for neu_adapter_t
-typedef struct neu_adapter neu_adapter_t;
-
 typedef enum {
     NEU_METRIC_TYPE_COUNTER,
     NEU_METRIC_TYPE_GAUAGE,