Add telemetry for continuous aggs

This adds a top-level telemetry entry for `num_continuous_aggs`.
2 designs were considered:
1) add entry to telemetry_metadata that's modified on add/drop
continuous aggs.
2) scan the catalogs for number of continuous aggs when making the
report.

After briefly trying (1), I switched to (2) for 2 reasons:
1) This avoids the dependency from continous_aggs on telemetry
and reverses that dependency. I think the other way is cleaner.
2) The code became simpler and more self-contained.

src/continuous_agg.c

@@ -595,3 +595,14 @@ ts_continuous_agg_rename_view(char *old_schema, char *name, char *new_schema, ch
 	}
 	return;
 }
+
+TSDLLEXPORT int32
+ts_number_of_continuous_aggs()
+{
+	int32 count = 0;
+	ScanIterator iterator =
+		ts_scan_iterator_create(CONTINUOUS_AGG, AccessShareLock, CurrentMemoryContext);
+	ts_scanner_foreach(&iterator) { count++; }
+
+	return count;
+}

src/continuous_agg.h

@@ -66,4 +66,6 @@ extern void ts_continuous_agg_rename_schema_name(char *old_schema, char *new_sch
 extern void ts_continuous_agg_rename_view(char *old_schema, char *name, char *new_schema,
 										  char *new_name);
 
+extern TSDLLEXPORT int32 ts_number_of_continuous_aggs(void);
+
 #endif /* TIMESCALEDB_TSL_CONTINUOUS_AGGS_CONTIGUOUS_AGG_H */

src/telemetry/telemetry.c

@@ -47,6 +47,7 @@
 #define REQ_BUILD_OS_VERSION "build_os_version"
 #define REQ_DATA_VOLUME "data_volume"
 #define REQ_NUM_HYPERTABLES "num_hypertables"
+#define REQ_NUM_CONTINUOUS_AGGS "num_continuous_aggs"
 #define REQ_RELATED_EXTENSIONS "related_extensions"
 #define REQ_TELEMETRY_METADATA "db_metadata"
 #define REQ_LICENSE_INFO "license"
@@ -163,6 +164,15 @@ get_num_hypertables()
 	return buf->data;
 }
 
+static char *
+get_num_continuous_aggs()
+{
+	StringInfo buf = makeStringInfo();
+
+	appendStringInfo(buf, "%d", ts_number_of_continuous_aggs());
+	return buf->data;
+}
+
 static char *
 get_database_size()
 {
@@ -252,6 +262,7 @@ build_version_body(void)
 	ts_jsonb_add_str(parseState, REQ_BUILD_OS_VERSION, BUILD_OS_VERSION);
 	ts_jsonb_add_str(parseState, REQ_DATA_VOLUME, get_database_size());
 	ts_jsonb_add_str(parseState, REQ_NUM_HYPERTABLES, get_num_hypertables());
+	ts_jsonb_add_str(parseState, REQ_NUM_CONTINUOUS_AGGS, get_num_continuous_aggs());
 
 	/* Add related extensions, which is a nested JSON */
 	ext_key.type = jbvString;

test/expected/telemetry.out

@@ -305,8 +305,15 @@ WHERE key != 'os_name_pretty';
  last_tuned_version
  postgresql_version
  related_extensions
+ num_continuous_aggs
  timescaledb_version
-(19 rows)
+(20 rows)
+
+SELECT json_object_field(get_telemetry_report()::json,'num_continuous_aggs');
+ json_object_field 
+-------------------
+ "0"
+(1 row)
 
 -- check telemetry picks up content from telemetry_metadata
 SELECT json_object_field(get_telemetry_report()::json,'db_metadata');
@@ -322,3 +329,34 @@ SELECT json_object_field(get_telemetry_report()::json,'instance_metadata');
  {"cloud": "ci"}
 (1 row)
 
+--create a continuous agg
+CREATE TABLE device_readings (
+      observation_time  TIMESTAMPTZ       NOT NULL
+);
+SELECT table_name FROM create_hypertable('device_readings', 'observation_time');
+   table_name    
+-----------------
+ device_readings
+(1 row)
+
+CREATE VIEW device_summary
+WITH (timescaledb.continuous) AS
+SELECT
+  time_bucket('1 hour', observation_time) as bucket,
+  min(observation_time)
+FROM
+  device_readings
+GROUP BY bucket;
+NOTICE:  adding not-null constraint to column "time_partition_col"
+SELECT json_object_field(get_telemetry_report()::json,'num_continuous_aggs');
+ json_object_field 
+-------------------
+ "1"
+(1 row)
+
+SELECT json_object_field(get_telemetry_report()::json,'num_hypertables');
+ json_object_field 
+-------------------
+ "2"
+(1 row)
+

test/sql/telemetry.sql

@@ -126,6 +126,8 @@ SET timescaledb.telemetry_level=off;
 SELECT * FROM json_object_keys(get_telemetry_report()::json) AS key
 WHERE key != 'os_name_pretty';
 
+SELECT json_object_field(get_telemetry_report()::json,'num_continuous_aggs');
+
 -- check telemetry picks up content from telemetry_metadata
 SELECT json_object_field(get_telemetry_report()::json,'db_metadata');
 
@@ -133,3 +135,19 @@ SELECT json_object_field(get_telemetry_report()::json,'db_metadata');
 SELECT json_object_field(get_telemetry_report()::json,'instance_metadata');
 
 
+--create a continuous agg
+CREATE TABLE device_readings (
+      observation_time  TIMESTAMPTZ       NOT NULL
+);
+SELECT table_name FROM create_hypertable('device_readings', 'observation_time');
+CREATE VIEW device_summary
+WITH (timescaledb.continuous) AS
+SELECT
+  time_bucket('1 hour', observation_time) as bucket,
+  min(observation_time)
+FROM
+  device_readings
+GROUP BY bucket;
+
+SELECT json_object_field(get_telemetry_report()::json,'num_continuous_aggs');
+SELECT json_object_field(get_telemetry_report()::json,'num_hypertables');