MINOR: stats: add functions to emit typed fields into a chunk

New function stats_emit_typed_data_field() does exactly like
stats_emit_raw_data_field() except that it also prints the data
type after a colon. This will be used to print using the typed
format.

And function stats_emit_field_tags() appends a 3-letter code
describing the origin, nature, and scope, followed by an optional
delimiter. This will be particularly convenient to dump typed
data.
diff --git a/include/proto/dumpstats.h b/include/proto/dumpstats.h
index 0fb3477..9080806 100644
--- a/include/proto/dumpstats.h
+++ b/include/proto/dumpstats.h
@@ -232,6 +232,9 @@
 extern struct applet http_stats_applet;
 
 void stats_io_handler(struct stream_interface *si);
+int stats_emit_raw_data_field(struct chunk *out, const struct field *f);
+int stats_emit_typed_data_field(struct chunk *out, const struct field *f);
+int stats_emit_field_tags(struct chunk *out, const struct field *f, char delim);
 
 
 #endif /* _PROTO_DUMPSTATS_H */