An enhancement to the APM implementation.

Here's how to get started. First, understand how I did the Command Monitoring implementation (CDRIVER-769), which is a precursor to this feature and has a very similar implementation to this one.

• Read the Command Monitoring Spec: https://github.com/mongodb/specifications/blob/master/source/command-monitoring/command-monitoring.rst
• Read application-performance-monitoring.page, mongoc-apm.h, and the monitoring-related parts of mongoc_cluster_run_command_internal, to understand how I implemented it.
• Look at a typical Command Monitoring test: https://github.com/mongodb/specifications/blob/master/source/command-monitoring/tests/insertOne.yml
• Read test-mongoc-command-monitoring.c, and in particular the test_all_spec_tests() function, which runs all the standard Command Monitoring tests.

Now read https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring-monitoring.rst

Now do similar work for SDAM Monitoring. Add to mongoc-apm.h and .c:

• Events: mongoc_apm_server_changed_t, mongoc_apm_server_opening_t, mongoc_apm_server_closed_t, mongoc_apm_topology_changed_t, mongoc_apm_topology_opening_t, mongoc_apm_topology_closed_t, mongoc_apm_server_heartbeat_started_t, mongoc_apm_server_heartbeat_succeeded_t, mongoc_apm_server_heartbeat_failed_t.
• Make event-field accessors for these 9 event structs: mongoc_apm_server_changed_get_address and so on. About 20 functions in all.
• Typedef the 9 callback function pointer types: mongoc_apm_server_changed_cb_t, mongoc_apm_server_opening_cb_t, mongoc_apm_server_closed_cb_t, mongoc_apm_cb_topology_changed_cb_t, mongoc_apm_cb_topology_opening_cb_t, mongoc_apm_cb_topology_closed_cb_t, mongoc_apm_server_heartbeat_started_cb_t, mongoc_apm_server_heartbeat_succeeded_cb_t, mongoc_apm_server_heartbeat_failed_cb_t
• Add 9 function pointers to _mongoc_apm_callbacks_t: _mongoc_apm_callbacks_t.server_changed, etc.
• Make 9 callback registrar functions: mongoc_apm_set_server_changed_cb, etc.
• Add mongoc_apm_callbacks_t* apm_callbacks field to topology description and server description.

Start testing and implementing the events.

• Adapt test-mongoc-command-monitoring.c for the SDAM Monitoring events.
• Copy the JSON tests from the specs repo.
• Disable all test asserts except for mongoc_apm_server_changed_t, then implement that event in mongoc_server_description_handle_ismaster?
• Enable test asserts for mongoc_apm_server_opening_t, implement in mongoc_topology_description_add_server.
• Enable test asserts for mongoc_apm_server_closed_t, implement in _mongoc_topology_description_remove_server.
• Enable test asserts for mongoc_apm_topology_changed_t, implement at the end of _mongoc_topology_scanner_cb?
• Enable test asserts for mongoc_apm_topology_opening_t, implement in mongoc_topology_description_init.
• Enable test asserts for mongoc_apm_topology_closed_t, implement in mongoc_topology_description_destroy.
• Enable test asserts for mongoc_apm_server_heartbeat_started_t, implement in new function mongoc_server_description_heartbeat_started, called from mongoc_topology_scanner_node_setup?
• Enable test asserts for mongoc_apm_server_heartbeat_succeeded_t, implement in mongoc_server_description_handle_ismaster.
• Enable test asserts for mongoc_apm_server_heartbeat_failed_t, implement in mongoc_server_description_handle_ismaster.

Finally, update application-performance-monitoring.page with an SDAM Monitoring example.