Use the pg_settings view to retrieve runtime variable

Adds all bool/integer/real runtime variables that can be retrieved ths way.

Use the `pg_settings` view to retrieve runtime variables:
https://www.postgresql.org/docs/current/static/view-pg-settings.html

Replaces the use of `SHOW` to retrieve runtime variables.

Only runtime variables with a `vartype` of `bool`, `real`, or `integer`
are currently supported. Uses the `short_desc` field as a description.

This commit deprecates the following metric names:
```
pg_runtime_variable_max_connections
pg_runtime_variable_max_files_per_process
pg_runtime_variable_max_function_args
pg_runtime_variable_max_identifier_length
pg_runtime_variable_max_index_keys
pg_runtime_variable_max_locks_per_transaction
pg_runtime_variable_max_pred_locks_per_transaction
pg_runtime_variable_max_prepared_transactions
pg_runtime_variable_max_standby_archive_delay_milliseconds
pg_runtime_variable_max_standby_streaming_delay_milliseconds
pg_runtime_variable_max_wal_senders
```

They are replaced by equivalent names under `pg_settings` with the exception of
```
pg_runtime_variable_max_standby_archive_delay_milliseconds
pg_runtime_variable_max_standby_streaming_delay_milliseconds
```
which are replaced with
```
pg_settings_max_standby_archive_delay_seconds
pg_settings_max_standby_streaming_delay_seconds
```

Adds approximately 195 new metrics, when considered across all supported
PostgreSQL versions.

pg_setting_test.go

@@ -0,0 +1,191 @@
+// +build !integration
+
+package main
+
+import (
+	dto "github.com/prometheus/client_model/go"
+	. "gopkg.in/check.v1"
+)
+
+type PgSettingSuite struct{}
+
+var _ = Suite(&PgSettingSuite{})
+
+var fixtures = []fixture{
+	fixture{
+		p: pgSetting{
+			name:      "seconds_fixture_metric",
+			setting:   "5",
+			unit:      "s",
+			shortDesc: "Foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  5,
+			unit: "seconds",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_seconds_fixture_metric_seconds\", help: \"Foo foo foo [Units converted to seconds.]\", constLabels: {}, variableLabels: []}",
+		v: 5,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "milliseconds_fixture_metric",
+			setting:   "5000",
+			unit:      "ms",
+			shortDesc: "Foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  5,
+			unit: "seconds",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_milliseconds_fixture_metric_seconds\", help: \"Foo foo foo [Units converted to seconds.]\", constLabels: {}, variableLabels: []}",
+		v: 5,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "eight_kb_fixture_metric",
+			setting:   "17",
+			unit:      "8kB",
+			shortDesc: "Foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  139264,
+			unit: "bytes",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_eight_kb_fixture_metric_bytes\", help: \"Foo foo foo [Units converted to bytes.]\", constLabels: {}, variableLabels: []}",
+		v: 139264,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "16_mb_real_fixture_metric",
+			setting:   "3.0",
+			unit:      "16MB",
+			shortDesc: "Foo foo foo",
+			vartype:   "real",
+		},
+		n: normalised{
+			val:  5.0331648e+07,
+			unit: "bytes",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_16_mb_real_fixture_metric_bytes\", help: \"Foo foo foo [Units converted to bytes.]\", constLabels: {}, variableLabels: []}",
+		v: 5.0331648e+07,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "bool_on_fixture_metric",
+			setting:   "on",
+			unit:      "",
+			shortDesc: "Foo foo foo",
+			vartype:   "bool",
+		},
+		n: normalised{
+			val:  1,
+			unit: "",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_bool_on_fixture_metric\", help: \"Foo foo foo\", constLabels: {}, variableLabels: []}",
+		v: 1,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "bool_off_fixture_metric",
+			setting:   "off",
+			unit:      "",
+			shortDesc: "Foo foo foo",
+			vartype:   "bool",
+		},
+		n: normalised{
+			val:  0,
+			unit: "",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_bool_off_fixture_metric\", help: \"Foo foo foo\", constLabels: {}, variableLabels: []}",
+		v: 0,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "special_minus_one_value",
+			setting:   "-1",
+			unit:      "d",
+			shortDesc: "foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  -1,
+			unit: "seconds",
+			err:  "",
+		},
+		d: "Desc{fqName: \"pg_settings_special_minus_one_value_seconds\", help: \"foo foo foo [Units converted to seconds.]\", constLabels: {}, variableLabels: []}",
+		v: -1,
+	},
+	fixture{
+		p: pgSetting{
+			name:      "unknown_unit",
+			setting:   "10",
+			unit:      "nonexistent",
+			shortDesc: "foo foo foo",
+			vartype:   "integer",
+		},
+		n: normalised{
+			val:  10,
+			unit: "",
+			err:  `Unknown unit for runtime variable: "nonexistent"`,
+		},
+	},
+}
+
+func (s *PgSettingSuite) TestNormaliseUnit(c *C) {
+	for _, f := range fixtures {
+		switch f.p.vartype {
+		case "integer", "real":
+			val, unit, err := f.p.normaliseUnit()
+
+			c.Check(val, Equals, f.n.val)
+			c.Check(unit, Equals, f.n.unit)
+
+			if err == nil {
+				c.Check("", Equals, f.n.err)
+			} else {
+				c.Check(err.Error(), Equals, f.n.err)
+			}
+		}
+	}
+}
+
+func (s *PgSettingSuite) TestMetric(c *C) {
+	defer func() {
+		if r := recover(); r != nil {
+			if r.(error).Error() != `Unknown unit for runtime variable: "nonexistent"` {
+				panic(r)
+			}
+		}
+	}()
+
+	for _, f := range fixtures {
+		d := &dto.Metric{}
+		m := f.p.metric()
+		m.Write(d)
+
+		c.Check(m.Desc().String(), Equals, f.d)
+		c.Check(d.GetGauge().GetValue(), Equals, f.v)
+	}
+}
+
+type normalised struct {
+	val  float64
+	unit string
+	err  string
+}
+
+type fixture struct {
+	p pgSetting
+	n normalised
+	d string
+	v float64
+}