Add sprig text/template function to template stage.

Fixes grafana#3129

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

docs/sources/clients/promtail/stages/template.md

@@ -113,6 +113,8 @@ The snippet above will for instance prepend the log line with the application na
 
 ## Supported Functions
 
+> All [sprig functions](http://masterminds.github.io/sprig/) have been added to the template stage in Loki 2.3 along with function described below.
+
 ### ToLower & ToUpper
 
 ToLower and ToUpper convert the entire string respectively to lowercase and uppercase.
@@ -201,7 +203,7 @@ and trailing white space removed, as defined by Unicode.
     template: '{{ Hash .Value "salt" }}'
 ```
 
-Alternatively, you can use `Sha2Hash` for calculating the Sha2_256 of the string. Sha2_256 is faster and requires less CPU than Sha3_256, however it is less secure. 
+Alternatively, you can use `Sha2Hash` for calculating the Sha2_256 of the string. Sha2_256 is faster and requires less CPU than Sha3_256, however it is less secure.
 
 We recommend using `Hash` as it has a stronger hashing algorithm which we plan to keep strong over time without requiring client config changes.
 

pkg/logentry/stages/template.go

@@ -11,6 +11,7 @@ import (
 	"text/template"
 	"time"
 
+	"github.com/Masterminds/sprig/v3"
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
 	"github.com/mitchellh/mapstructure"
@@ -25,35 +26,41 @@ const (
 	ErrTemplateSourceRequired   = "template source value is required"
 )
 
-var (
-	functionMap = template.FuncMap{
-		"ToLower":    strings.ToLower,
-		"ToUpper":    strings.ToUpper,
-		"Replace":    strings.Replace,
-		"Trim":       strings.Trim,
-		"TrimLeft":   strings.TrimLeft,
-		"TrimRight":  strings.TrimRight,
-		"TrimPrefix": strings.TrimPrefix,
-		"TrimSuffix": strings.TrimSuffix,
-		"TrimSpace":  strings.TrimSpace,
-		"Hash": func(salt string, input string) string {
-			hash := sha3.Sum256([]byte(salt + input))
-			return hex.EncodeToString(hash[:])
-		},
-		"Sha2Hash": func(salt string, input string) string {
-			hash := sha256.Sum256([]byte(salt + input))
-			return hex.EncodeToString(hash[:])
-		},
-		"regexReplaceAll": func(regex string, s string, repl string) string {
-			r := regexp.MustCompile(regex)
-			return r.ReplaceAllString(s, repl)
-		},
-		"regexReplaceAllLiteral": func(regex string, s string, repl string) string {
-			r := regexp.MustCompile(regex)
-			return r.ReplaceAllLiteralString(s, repl)
-		},
+var extraFunctionMap = template.FuncMap{
+	"ToLower":    strings.ToLower,
+	"ToUpper":    strings.ToUpper,
+	"Replace":    strings.Replace,
+	"Trim":       strings.Trim,
+	"TrimLeft":   strings.TrimLeft,
+	"TrimRight":  strings.TrimRight,
+	"TrimPrefix": strings.TrimPrefix,
+	"TrimSuffix": strings.TrimSuffix,
+	"TrimSpace":  strings.TrimSpace,
+	"Hash": func(salt string, input string) string {
+		hash := sha3.Sum256([]byte(salt + input))
+		return hex.EncodeToString(hash[:])
+	},
+	"Sha2Hash": func(salt string, input string) string {
+		hash := sha256.Sum256([]byte(salt + input))
+		return hex.EncodeToString(hash[:])
+	},
+	"regexReplaceAll": func(regex string, s string, repl string) string {
+		r := regexp.MustCompile(regex)
+		return r.ReplaceAllString(s, repl)
+	},
+	"regexReplaceAllLiteral": func(regex string, s string, repl string) string {
+		r := regexp.MustCompile(regex)
+		return r.ReplaceAllLiteralString(s, repl)
+	},
+}
+
+var functionMap = sprig.TxtFuncMap()
+
+func init() {
+	for k, v := range extraFunctionMap {
+		functionMap[k] = v
 	}
-)
+}
 
 // TemplateConfig configures template value extraction
 type TemplateConfig struct {
@@ -135,7 +142,6 @@ func (o *templateStage) Process(labels model.LabelSet, extracted map[string]inte
 	} else {
 		extracted[o.cfgs.Source] = st
 	}
-
 }
 
 // Name implements Stage

pkg/logentry/stages/template_test.go

@@ -226,6 +226,18 @@ func TestTemplateStage_Process(t *testing.T) {
 				"testval": "value",
 			},
 		},
+		"sprig": {
+			TemplateConfig{
+				Source:   "testval",
+				Template: "{{ add 7 3 }}",
+			},
+			map[string]interface{}{
+				"testval": "Value",
+			},
+			map[string]interface{}{
+				"testval": "10",
+			},
+		},
 		"ToLowerParams": {
 			TemplateConfig{
 				Source:   "testval",