Introduce histogram support (#435)

* Introduce histogram support

Prior to this change, the custom queries were restricted to counters and
gauges.

This change introduces a new ColumnUsage, namely HISTOGRAM, that expects
the column to contain an array of upper inclusive bounds for each
observation bucket in the emitted metric.  It also expects three more
columns to be present with the suffixes:
- `_bucket`, containing an array of cumulative counters for the
  observation buckets;
- `_sum`, the total sum of all observed values; and
- `_count`, the count of events that have been observed.

A flag has been added to the MetricMap struct to easily identify metrics
that should emit a histogram and the construction of a histogram metric
is aided by the pg.Array function and a new helper dbToUint64 function.

Finally, and example of usage is given in queries.yaml.

fixes #402

Signed-off-by: Corin Lawson <corin@responsight.com>

* Introduces tests for histogram support

Prior to this change, the histogram support was untested.

This change introduces a new integration test that reads a user query
containing a number of histogram metrics.  Also, additional checks have
been added to TestBooleanConversionToValueAndString to test dbToUint64.

Signed-off-by: Corin Lawson <corin@responsight.com>

queries.yaml

@@ -228,3 +228,47 @@ pg_stat_statements:
     - blk_write_time_seconds:
         usage: "COUNTER"
         description: "Total time the statement spent writing blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)"
+
+pg_stat_activity:
+  query: |
+    WITH
+      metrics AS (
+        SELECT
+          application_name,
+          SUM(EXTRACT(EPOCH FROM (CURRENT_TIMESTAMP - state_change))::bigint)::float AS process_idle_seconds_sum,
+          COUNT(*) AS process_idle_seconds_count
+        FROM pg_stat_activity
+        WHERE state = 'idle'
+        GROUP BY application_name
+      ),
+      buckets AS (
+        SELECT
+          application_name,
+          le,
+          SUM(
+            CASE WHEN EXTRACT(EPOCH FROM (CURRENT_TIMESTAMP - state_change)) <= le
+              THEN 1
+              ELSE 0
+            END
+          )::bigint AS bucket
+        FROM
+          pg_stat_activity,
+          UNNEST(ARRAY[1, 2, 5, 15, 30, 60, 90, 120, 300]) AS le
+        GROUP BY application_name, le
+        ORDER BY application_name, le
+      )
+    SELECT
+      application_name,
+      process_idle_seconds_sum,
+      process_idle_seconds_count,
+      ARRAY_AGG(le) AS process_idle_seconds,
+      ARRAY_AGG(bucket) AS process_idle_seconds_bucket
+    FROM metrics JOIN buckets USING (application_name)
+    GROUP BY 1, 2, 3
+  metrics:
+    - application_name:
+        usage: "LABEL"
+        description: "Application Name"
+    - process_idle_seconds:
+        usage: "HISTOGRAM"
+        description: "Idle time of server processes"