internal/telemetry/event: fix error/value key type tag formatting

%v behaves better with nil values.

Change-Id: I4041f5460173c8f420553996d64d9463ef5a0370
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227355
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: hyangah

internal/telemetry/event/tag.go

@@ -106,9 +106,9 @@ func (t Tag) Format(f fmt.State, r rune) {
 	case *StringKey:
 		fmt.Fprintf(f, "%s=%q", key.Name(), key.From(t))
 	case *ErrorKey:
-		fmt.Fprintf(f, "%s=%q", key.Name(), key.From(t))
+		fmt.Fprintf(f, "%s=%v", key.Name(), key.From(t))
 	case *ValueKey:
-		fmt.Fprintf(f, "%s=%q", key.Name(), key.From(t))
+		fmt.Fprintf(f, "%s=%v", key.Name(), key.From(t))
 	default:
 		fmt.Fprintf(f, `%s="invalid type %T"`, key.Name(), key)
 	}