Add left function to string processor (influxdata#6189)

plugins/processors/strings/README.md

@@ -11,6 +11,7 @@ Implemented functions are:
 - trim_prefix
 - trim_suffix
 - replace
+- left
 
 Please note that in this implementation these are processed in the order that they appear above.
 
@@ -62,6 +63,11 @@ If you'd like to apply multiple processings to the same `tag_key` or `field_key`
   #   measurement = "*"
   #   old = ":"
   #   new = "_"
+
+  ## Trims strings based on width
+  # [[processors.strings.left]]
+  #   field = "message"
+  #   width = 10
 ```
 
 #### Trim, TrimLeft, TrimRight

plugins/processors/strings/strings.go

@@ -17,6 +17,7 @@ type Strings struct {
 	TrimPrefix []converter `toml:"trim_prefix"`
 	TrimSuffix []converter `toml:"trim_suffix"`
 	Replace    []converter `toml:"replace"`
+	Left       []converter `toml:"left"`
 
 	converters []converter
 	init       bool
@@ -36,6 +37,7 @@ type converter struct {
 	Prefix      string
 	Old         string
 	New         string
+	Width       int
 
 	fn ConvertFunc
 }
@@ -79,6 +81,11 @@ const sampleConfig = `
   #   measurement = "*"
   #   old = ":"
   #   new = "_"
+
+  ## Trims strings based on width
+  # [[processors.strings.left]]
+  #   field = "message"
+  #   width = 10
 `
 
 func (s *Strings) SampleConfig() string {
@@ -270,6 +277,17 @@ func (s *Strings) initOnce() {
 		}
 		s.converters = append(s.converters, c)
 	}
+	for _, c := range s.Left {
+		c := c
+		c.fn = func(s string) string {
+			if len(s) < c.Width {
+				return s
+			} else {
+				return s[:c.Width]
+			}
+		}
+		s.converters = append(s.converters, c)
+	}
 
 	s.init = true
 }

plugins/processors/strings/strings_test.go

@@ -479,6 +479,38 @@ func TestFieldKeyConversions(t *testing.T) {
 				require.Equal(t, "/mixed/CASE/paTH/?from=-1D&to=now", fv)
 			},
 		},
+		{
+			name: "Should trim the existing field to 6 characters",
+			plugin: &Strings{
+				Left: []converter{
+					{
+						Field: "Request",
+						Width: 6,
+					},
+				},
+			},
+			check: func(t *testing.T, actual telegraf.Metric) {
+				fv, ok := actual.GetField("Request")
+				require.True(t, ok)
+				require.Equal(t, "/mixed", fv)
+			},
+		},
+		{
+			name: "Should do nothing to the string",
+			plugin: &Strings{
+				Left: []converter{
+					{
+						Field: "Request",
+						Width: 600,
+					},
+				},
+			},
+			check: func(t *testing.T, actual telegraf.Metric) {
+				fv, ok := actual.GetField("Request")
+				require.True(t, ok)
+				require.Equal(t, "/mixed/CASE/paTH/?from=-1D&to=now", fv)
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {