exec plugin: allow using glob pattern in command list

Allow using glob pattern in the command list in configuration. This enables for
example placing all commands in a single directory and using /path/to/dir/*.sh
as one of the commands to run all shell scripts in that directory.

Glob patterns are applied on every run of the commands, so matching commands can
be added without restarting telegraf.

closes #1142

plugins/inputs/exec/exec_test.go

@@ -169,3 +169,51 @@ func TestLineProtocolParseMultiple(t *testing.T) {
 		acc.AssertContainsTaggedFields(t, "cpu", fields, tags)
 	}
 }
+
+func TestExecCommandWithGlob(t *testing.T) {
+	parser, _ := parsers.NewValueParser("metric", "string", nil)
+	e := NewExec()
+	e.Commands = []string{"/bin/ech* metric_value"}
+	e.SetParser(parser)
+
+	var acc testutil.Accumulator
+	err := e.Gather(&acc)
+	require.NoError(t, err)
+
+	fields := map[string]interface{}{
+		"value": "metric_value",
+	}
+	acc.AssertContainsFields(t, "metric", fields)
+}
+
+func TestExecCommandWithoutGlob(t *testing.T) {
+	parser, _ := parsers.NewValueParser("metric", "string", nil)
+	e := NewExec()
+	e.Commands = []string{"/bin/echo metric_value"}
+	e.SetParser(parser)
+
+	var acc testutil.Accumulator
+	err := e.Gather(&acc)
+	require.NoError(t, err)
+
+	fields := map[string]interface{}{
+		"value": "metric_value",
+	}
+	acc.AssertContainsFields(t, "metric", fields)
+}
+
+func TestExecCommandWithoutGlobAndPath(t *testing.T) {
+	parser, _ := parsers.NewValueParser("metric", "string", nil)
+	e := NewExec()
+	e.Commands = []string{"echo metric_value"}
+	e.SetParser(parser)
+
+	var acc testutil.Accumulator
+	err := e.Gather(&acc)
+	require.NoError(t, err)
+
+	fields := map[string]interface{}{
+		"value": "metric_value",
+	}
+	acc.AssertContainsFields(t, "metric", fields)
+}