To clarify even more: that length is the number of bytes (or UTF-8 code units) in the string. It doesn't corresponding to the number of characters (which one may either consider to be Unicode codepoints, or more technically correct, Unicode grapheme clusters).
If you want to count the number of codepoints in a string (called "rune" in Go), then you need to do so explicitly: https://golang.org/pkg/unicode/utf8/#RuneCountInString