[v15] Update metrics.mdx with better clarity for heartbeat_connection…

…s_received_total (#43216)

* Update metrics.mdx with better clarity for heartbeat_connections_received_total

adding clarity to heartbeat_connections_received_total due to confusion around what a heartbeat connection represented. 

Confirmed that the metric counts the number of times an agent has connected to the endpoint on auth used to periodically send heartbeats. So it represents the number of agents actively heartbeating rather than the number of heartbeats received over the lifespan of that particular auth instance.

* Update docs/pages/includes/metrics.mdx

Co-authored-by: Paul Gottschling <paul.gottschling@goteleport.com>

* Update metrics.mdx

change heartbeating to heart beating

---------

Co-authored-by: Paul Gottschling <paul.gottschling@goteleport.com>

docs/pages/includes/metrics.mdx

@@ -46,7 +46,7 @@
 | `grpc_server_handled_total`                              | counter   | Teleport Auth       | Total number of RPCs completed on the server, regardless of success or failure.                    |
 | `grpc_server_msg_received_total`                         | counter   | Teleport Auth       | Total number of RPC stream messages received on the server.                                        |
 | `grpc_server_msg_sent_total`                             | counter   | Teleport Auth       | Total number of gRPC stream messages sent by the server.                                           |
-| `heartbeat_connections_received_total`                   | counter   | Teleport Auth       | Number of times the Auth Service received a heartbeat connection.                                  |
+| `heartbeat_connections_received_total`                   | counter   | Teleport Auth       | Number of times the Auth Service received a heartbeat connection, representing total heart beating Agents. |
 | `s3_requests_total`                                      | counter   | Amazon S3           | Total number of requests to the S3 API.                                                            |
 | `s3_requests`                                            | counter   | Amazon S3           | Total number of requests to the S3 API grouped by result.                                          |
 | `s3_requests_seconds`                                    | histogram | Amazon S3           | Request latency for the S3 API.                                                                    |