Speed up ros_topic_name_to_zenoh_key.

Use stringstream here is really slow; we can speed it
up by about 100% by switching to std::to_string instead.

Signed-off-by: Chris Lalancette <[email protected]>

rmw_zenoh_cpp/src/rmw_zenoh.cpp

@@ -25,7 +25,6 @@
 #include <new>
 #include <optional>
 #include <random>
-#include <sstream>
 #include <string>
 #include <utility>
 
@@ -82,6 +81,8 @@ namespace
 z_owned_keyexpr_t ros_topic_name_to_zenoh_key(
   const char * const topic_name, size_t domain_id, rcutils_allocator_t * allocator)
 {
+  std::string d = std::to_string(domain_id);
+
   size_t start_offset = 0;
   size_t topic_name_len = strlen(topic_name);
   size_t end_offset = topic_name_len;
@@ -97,18 +98,17 @@ z_owned_keyexpr_t ros_topic_name_to_zenoh_key(
     }
   }
 
-  std::stringstream domain_ss;
-  domain_ss << domain_id;
   char * stripped_topic_name = rcutils_strndup(
     &topic_name[start_offset], end_offset - start_offset, *allocator);
   if (stripped_topic_name == nullptr) {
     return z_keyexpr_null();
   }
-  z_owned_keyexpr_t keyexpr = z_keyexpr_join(
-    z_keyexpr(domain_ss.str().c_str()), z_keyexpr(stripped_topic_name));
+
+  z_owned_keyexpr_t ret = z_keyexpr_join(z_keyexpr(d.c_str()), z_keyexpr(stripped_topic_name));
+
   allocator->deallocate(stripped_topic_name, allocator->state);
 
-  return keyexpr;
+  return ret;
 }
 
 //==============================================================================