[CrateDB] Improve standard SQL statement for Grafana instant dashboards

Apply time bucketing over interval obtained from Grafana's date range
picker.

Emulate `GROUP BY DATE_BIN()` by using Grafana's `$__timeGroupAlias`
macro for casting `$__interval` values, until CrateDB's `DATE_BIN()`
function understands Grafana's native interval values.

kotori/daq/graphing/grafana/resources/grafana-target-cratedb.json

@@ -6,5 +6,5 @@
     "groupByTags": [],
     "measurement": "{{ measurement }}",
     "rawQuery": true,
-    "rawSql": "SELECT time, fields['{{ name }}'] AS {{ alias }} FROM {{ table }} WHERE $__timeFilter(time)"
+    "rawSql": "SELECT $__timeGroupAlias(time, $__interval), MEAN(fields['{{ name }}']) AS {{ alias }} FROM {{ table }} WHERE $__timeFilter(time) GROUP BY time ORDER BY time"
 }

test/test_daq_grafana.py

@@ -78,7 +78,9 @@ def test_mqtt_cratedb_grafana_single(machinery_cratedb, reset_cratedb, reset_gra
     logger.info('Grafana: Checking dashboard layout')
     target = dashboard['rows'][0]['panels'][0]['targets'][0]
     assert target['measurement'] == settings.cratedb_measurement_sensors
-    assert target['rawSql'] == "SELECT time, fields['humidity'] AS humidity FROM mqttkit_2_itest.foo_bar_sensors WHERE $__timeFilter(time)"
+    assert target['rawSql'] == \
+           "SELECT $__timeGroupAlias(time, $__interval, 0), MEAN(fields['humidity']) AS humidity " \
+           "FROM mqttkit_2_itest.foo_bar_sensors WHERE $__timeFilter(time) GROUP BY time ORDER BY time"
 
 
 @pytest_twisted.inlineCallbacks