char read() {
            checkArgument(hasNext(), "path: %s (out of range at index %s)", path(), index());
            return path.charAt(index++);
        }
Parses 'Variable' part.

<p>
Variable = "{" FieldPath [ "=" Segments ] "}"
</p>
grpc/src/main/java/com/linecorp/armeria/server/grpc/HttpJsonTranscodingPathParser.java