res_prometheus: Add Asterisk channel metrics

This patch adds basic Asterisk channel statistics to the res_prometheus
module. This includes:

* asterisk_calls_sum: A running sum of the total number of
  processed calls

* asterisk_calls_count: The current number of calls

* asterisk_channels_count: The current number of channels

* asterisk_channels_state: The state of any particular channel

* asterisk_channels_duration_seconds: How long a channel has existed,
  in seconds

In all cases, enough information is provided with each channel metric
to determine a unique instance of Asterisk that provided the data, as
well as the name, type, unique ID, and - if present - linked ID of each
channel.

ASTERISK-28403

Change-Id: I0db306ec94205d4f58d1e7fbabfe04b185869f59

include/asterisk/res_prometheus.h

@@ -83,6 +83,36 @@ struct prometheus_general_config {
 	);
 };
 
+/*!
+ * \brief A function table for a metrics provider
+ *
+ * \details
+ * It's generally nice to separate out things that provide metrics
+ * from the core of this module. For those that want to be notified
+ * when things happen in the core module, they can provide an instance
+ * of this function table using \c prometheus_metrics_provider_register
+ * and be notified when module affecting changes occur.
+ */
+struct prometheus_metrics_provider {
+	/*!
+	 * \brief Handy name of the provider for debugging purposes
+	 */
+	const char *name;
+	/*!
+	 * \brief Reload callback
+	 *
+	 * \param config The reloaded config
+	 *
+	 * \retval 0 success
+	 * \retval -1 error
+	 */
+	int (* const reload_cb)(struct prometheus_general_config *config);
+	/*!
+	 * \brief Unload callback.
+	 */
+	void (* const unload_cb)(void);
+};
+
 /*!
  * \brief Prometheus metric type
  *
@@ -437,6 +467,13 @@ int prometheus_callback_register(struct prometheus_callback *callback);
  */
 void prometheus_callback_unregister(struct prometheus_callback *callback);
 
+/*!
+ * \brief Register a metrics provider
+ *
+ * \param provider The provider function table to register
+ */
+void prometheus_metrics_provider_register(const struct prometheus_metrics_provider *provider);
+
 /*!
  * \brief Retrieve the current configuration of the module
  *