Methods to retrieve matched counts on publisher and subscriber (#155)

* Methods to retrieve matched counts on pub/sub.

* Address reviewer feedback with naming and docs.

* Fix typos.

* Apply suggestions from code review

Co-Authored-By: mjcarroll <[email protected]>

rmw/include/rmw/rmw.h

@@ -188,6 +188,24 @@ RMW_WARN_UNUSED
 rmw_ret_t
 rmw_publish(const rmw_publisher_t * publisher, const void * ros_message);
 
+/// Retrieve the number of matched subscriptions to a publisher.
+/**
+ * Query the underlying middleware to determine how many subscriptions are
+ * matched to a given publisher.
+ *
+ * \param[in] publisher the publisher object to inspect
+ * \param[out] subscription_count the number of subscriptions matched
+ * \return `RMW_RET_OK` if successful, or
+ * \return `RMW_RET_INVALID_ARGUMENT` if either argument is null, or
+ * \return `RMW_RET_ERROR` if an unexpected error occurs.
+ */
+RMW_PUBLIC
+RMW_WARN_UNUSED
+rmw_ret_t
+rmw_publisher_count_matched_subscriptions(
+  const rmw_publisher_t * publisher,
+  size_t * subscription_count);
+
 /// Publish an already serialized message.
 /**
  * The publisher must already be registered with the correct message type
@@ -267,6 +285,24 @@ RMW_WARN_UNUSED
 rmw_ret_t
 rmw_destroy_subscription(rmw_node_t * node, rmw_subscription_t * subscription);
 
+/// Retrieve the number of matched publishers to a subscription.
+/**
+ * Query the underlying middleware to determine how many publishers are
+ * matched to a given subscription.
+ *
+ * \param[in] subscription the subscription object to inspect
+ * \param[out] publisher_count the number of publishers matched
+ * \return `RMW_RET_OK` if successful, or
+ * \return `RMW_RET_INVALID_ARGUMENT` if either argument is null, or
+ * \return `RMW_RET_ERROR` if an unexpected error occurs.
+ */
+RMW_PUBLIC
+RMW_WARN_UNUSED
+rmw_ret_t
+rmw_subscription_count_matched_publishers(
+  const rmw_subscription_t * subscription,
+  size_t * publisher_count);
+
 RMW_PUBLIC
 RMW_WARN_UNUSED
 rmw_ret_t