Add a way to add process ID to file names

When you use S3 and write from multiple machines, you need to ensure that file will have unique name for each machine.
This PR adds special %i function which you can use in file templates, which will add some random process uniq string to the file.

output_file.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"math/rand"
 	"os"
 	"path/filepath"
 	"runtime/debug"
@@ -19,6 +20,21 @@ import (
 	"github.com/buger/goreplay/size"
 )
 
+var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
+var instanceID string
+
+func init() {
+	instanceID = randSeq(8)
+}
+
+func randSeq(n int) string {
+	b := make([]rune, n)
+	for i := range b {
+		b[i] = letters[rand.Intn(len(letters))]
+	}
+	return string(b)
+}
+
 var dateFileNameFuncs = map[string]func(*FileOutput) string{
 	"%Y":  func(o *FileOutput) string { return time.Now().Format("2006") },
 	"%m":  func(o *FileOutput) string { return time.Now().Format("01") },
@@ -29,6 +45,7 @@ var dateFileNameFuncs = map[string]func(*FileOutput) string{
 	"%NS": func(o *FileOutput) string { return fmt.Sprint(time.Now().Nanosecond()) },
 	"%r":  func(o *FileOutput) string { return string(o.currentID) },
 	"%t":  func(o *FileOutput) string { return string(o.payloadType) },
+	"%i":  func(o *FileOutput) string { return instanceID },
 }
 
 // FileOutputConfig ...