Add date and tz functions to templates

[grobinson-grafana]

This commit adds the date and tz functions to templates. This means users can now format time in a specified format and also change the timezone to their specific locale.

An example of how these functions work, and can be composed together, can be seen here:

{{ .StartsAt | tz "Europe/Paris" | date "15:04:05 MST" }}

[verdel]

Perhaps we should add a duration function? This would, for example, allow generating links to a grafana dashboards where a time range for display can be passed.

We will pass a time offset in the annotation and use it to generate the link in the following way:

"annotations": {
        "title": "Alert Summary text",
        "description": "Detailed alert description text john doe jane doe some veeery long text needed",
        "log_url": "https://grafana.example.com ...from=%from%, to=%to%",
        "grafana_dashboard_range": "-2m"
      },

{{ define "test" }}
{{ .Annotations.message }}
{{- "\\n" -}}
{{ (index .Alerts 0).Annotations.log_url | reReplaceAll "%from%" (printf "%d000" ((index .Alerts 0).StartsAt.Add (duration (index .Alerts 0).Annotations.grafana_dashboard_range)).Unix) | reReplaceAll "%to%" (printf "%d000" (index .Alerts 0).StartsAt.Unix)}}
{{ end }}

The function itself is very simple:

// duration returns the time.Duration representation of a given string
"duration": func(text string) time.Duration {
	d, _ := time.ParseDuration(text)
	return d
}

[grobinson-grafana]

@verdel can you open a separate issue for that?

[gotjosh]

Can you please add unhappy cases? I'm not sure what happens when you pass a non-valid format to date or when empty value time and an error.

[grobinson-grafana]

Yeah sure! Template panics like other functions. As far as I can tell, there is no discernible difference between returning an error and panicking in the function, both result in a panic:

"reReplaceAll": func(pattern, repl, text string) string {
	re := regexp.MustCompile(pattern) // panics if regex is invalid
	return re.ReplaceAllString(text, repl)
},

// tz returns the time in the timezone.
"tz": func(name string, t time.Time) (time.Time, error) {
	loc, err := time.LoadLocation(name)
	if err != nil {
		return time.Time{}, err // panics with err
	}
	return t.In(loc), nil
},

[gotjosh]

LGTM