Add parseDuration to template functions

This can be useful when generating rules, a query may use a duration,
and it may be useful to template that into a URL parameter. Therefore
this allows interfacing with systems that don't implement Prometheus
style duration parsing.

Signed-off-by: David Leadbeater <dgl@dgl.cx>

docs/configuration/template_reference.md

@@ -74,6 +74,7 @@ versions.
 | reReplaceAll  | pattern, replacement, text | string | [Regexp.ReplaceAllString](https://golang.org/pkg/regexp/#Regexp.ReplaceAllString) Regexp substitution, unanchored. |
 | graphLink  | expr | string | Returns path to graph view in the [expression browser](https://prometheus.io/docs/visualization/browser/) for the expression. |
 | tableLink  | expr | string | Returns path to tabular ("Table") view in the [expression browser](https://prometheus.io/docs/visualization/browser/) for the expression. |
+| parseDuration | string | string | Parses a duration string such as "1h" into the integer number of seconds it represents. |
 
 ### Others
 

template/template.go

@@ -304,6 +304,14 @@ func NewTemplateExpander(
 			"externalURL": func() string {
 				return externalURL.String()
 			},
+			"parseDuration": func(d string) (string, error) {
+				v, err := model.ParseDuration(d)
+				if err != nil {
+					return "", err
+				}
+				dur := int64(time.Duration(v) / time.Second)
+				return fmt.Sprintf("%d", dur), nil
+			},
 		},
 		options: options,
 	}

template/template_test.go

@@ -449,6 +449,11 @@ func TestTemplateExpansion(t *testing.T) {
 			text:   "{{ externalURL }}",
 			output: "http://testhost:9090/path/prefix",
 		},
+		{
+			// parseDuration.
+			text:   "{{ parseDuration \"1h2m10ms\" }}",
+			output: "3720",
+		},
 	}
 
 	extURL, err := url.Parse("http://testhost:9090/path/prefix")