Add template functions to support various use cases. (#16619)

Presumably, this will help with Loki alerts, but the added functionality is also generally useful.

For one, this enables `parseDuration` to also accept negative duration (as that's something that is also used in PromQL by now).

This also adds a function `now` to return the evaluation time of the template (as seconds since epoch AKA Unix time) and a function `toDuration` (akin to `toTime`), which creates a Go `time.Duration` from a duration in seconds.

---------

Signed-off-by: Dmitry Ponomaryov <me@halje.ru>
Signed-off-by: Dmitry Ponomaryov <iamhalje@gmail.com>

docs/configuration/template_reference.md

@@ -57,6 +57,8 @@ If functions are used in a pipeline, the pipeline value is passed as the last ar
 | humanizePercentage  | number or string | string  | Converts a ratio value to a fraction of 100. |
 | humanizeTimestamp   | number or string | string         | Converts a Unix timestamp in seconds to a more readable format. |
 | toTime              | number or string | *time.Time     | Converts a Unix timestamp in seconds to a time.Time.            |
+| toDuration          | number or string | *time.Duration | Converts a duration in seconds to a time.Duration. |
+| now                 | none             | float64        | Returns the Unix timestamp in seconds at the time of the template evaluation. |
 
 Humanizing functions are intended to produce reasonable output for consumption
 by humans, and are not guaranteed to return the same results between Prometheus

template/template.go

@@ -263,6 +263,17 @@ func NewTemplateExpander(
 
 				return floatToTime(v)
 			},
+			"toDuration": func(i interface{}) (*time.Duration, error) {
+				v, err := common_templates.ConvertToFloat(i)
+				if err != nil {
+					return nil, err
+				}
+				d := time.Duration(v * float64(time.Second))
+				return &d, nil
+			},
+			"now": func() float64 {
+				return float64(timestamp) / 1000.0
+			},
 			"pathPrefix": func() string {
 				return externalURL.Path
 			},
@@ -270,7 +281,7 @@ func NewTemplateExpander(
 				return externalURL.String()
 			},
 			"parseDuration": func(d string) (float64, error) {
-				v, err := model.ParseDuration(d)
+				v, err := model.ParseDurationAllowNegative(d)
 				if err != nil {
 					return 0, err
 				}

template/template_test.go

@@ -21,6 +21,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/require"
 
 	"github.com/prometheus/prometheus/model/histogram"
@@ -467,6 +468,31 @@ func TestTemplateExpansion(t *testing.T) {
 			text:   `{{ ("1435065584.128" | toTime).Format "2006" }}`,
 			output: "2015",
 		},
+		{
+			// toDuration - input as float64 seconds, returns *time.Duration.
+			text:   `{{ (1800 | toDuration).String }}`,
+			output: "30m0s",
+		},
+		{
+			// toDuration - input as string seconds, returns *time.Duration.
+			text:   `{{ ("1800" | toDuration).String }}`,
+			output: "30m0s",
+		},
+		{
+			// now - returns fixed timestamp as float64 seconds.
+			text:   `{{ now }}`,
+			output: "1.353755652e+09",
+		},
+		{
+			// now - returns fixed timestamp converted to formatted time string.
+			text:   `{{ (now | toTime).Format "Mon Jan 2 15:04:05 2006" }}`,
+			output: "Sat Nov 24 11:14:12 2012",
+		},
+		{
+			// returns Unix milliseconds timestamp for 30 minutes ago.
+			text:   `{{ ("-30m" | parseDuration | toDuration | (now | toTime).Add).UnixMilli }}`,
+			output: "1353753852000",
+		},
 		{
 			// Title.
 			text:   "{{ \"aa bb CC\" | title }}",
@@ -514,10 +540,15 @@ func TestTemplateExpansion(t *testing.T) {
 			output: "http://testhost:9090/path/prefix",
 		},
 		{
-			// parseDuration (using printf to ensure the return is a string).
+			// parseDuration with positive duration (using printf to ensure the return is a string).
 			text:   "{{ printf \"%0.2f\" (parseDuration \"1h2m10ms\") }}",
 			output: "3720.01",
 		},
+		{
+			// parseDuration with negative duration (using printf to ensure the return is a string).
+			text:   "{{ printf \"%0.2f\" (parseDuration \"-1h2m10ms\") }}",
+			output: "-3720.01",
+		},
 		{
 			// Simple hostname.
 			text:   "{{ \"foo.example.com\" | stripDomain }}",
@@ -579,7 +610,7 @@ func testTemplateExpansion(t *testing.T, scenarios []scenario) {
 		}
 		var result string
 		var err error
-		expander := NewTemplateExpander(context.Background(), s.text, "test", s.input, 0, queryFunc, extURL, s.options)
+		expander := NewTemplateExpander(context.Background(), s.text, "test", s.input, model.Time(1353755652000), queryFunc, extURL, s.options)
 		if s.html {
 			result, err = expander.ExpandHTML(nil)
 		} else {