Merge mssql and dfsr expandEnabledCollectors func

Move to common collector.go and add function test.

Signed-off-by: Ben Reedy <breed808@breed808.com>

collector/collector.go

@@ -2,6 +2,7 @@ package collector
 
 import (
 	"fmt"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -128,3 +129,22 @@ func find(slice []string, val string) bool {
 	}
 	return false
 }
+
+// Used by more complex collectors where user input specifies enabled child collectors.
+// Splits provided child collectors and deduplicate.
+func expandEnabledChildCollectors(enabled string) []string {
+	separated := strings.Split(enabled, ",")
+	unique := map[string]bool{}
+	for _, s := range separated {
+		if s != "" {
+			unique[s] = true
+		}
+	}
+	result := make([]string, 0, len(unique))
+	for s := range unique {
+		result = append(result, s)
+	}
+	// Ensure result is ordered, to prevent test failure
+	sort.Strings(result)
+	return result
+}