feat(spanner): make randIDForProcess hexadecimal in x-goog-spanner-request-id (googleapis#11546)

Implements a new requirement that randIDForProcess should
be a hexadecimal value with the format specifier "%016x".

Fixes googleapis#11545

Author: odeke-em

spanner/request_id_header.go

@@ -46,7 +46,7 @@ func init() {
 	if err != nil {
 		panic(err)
 	}
-	randIDForProcess = r64.String()
+	randIDForProcess = fmt.Sprintf("%016x", r64.Uint64())
 }
 
 // Please bump this version whenever this implementation

spanner/request_id_header_test.go

@@ -16,6 +16,7 @@ package spanner
 
 import (
 	"context"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"math"
@@ -1925,3 +1926,13 @@ func TestRequestIDHeader_SingleUseReadOnly_ExecuteStreamingSql_UnavailableDuring
 		t.Fatalf("RequestID streaming segments mismatch: got - want +\n%s", diff)
 	}
 }
+
+func TestRequestID_randIDForProcessIsHexadecimal(t *testing.T) {
+	decoded, err := hex.DecodeString(randIDForProcess)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(decoded) == 0 {
+		t.Fatal("Expected a non-empty decoded result")
+	}
+}