Increments the number stored at `key` by `increment`. If the key does not exist,
it is set to `0` before performing the operation. An error is returned if the
key contains a value of the wrong type or contains a string that can not be
represented as integer. This operation is limited to 64 bit signed integers.

See `INCR` for extra information on increment/decrement operations.

@return

@integer-reply: the value of `key` after the increment

@examples

```cli
SET mykey "10"
INCRBY mykey 5
```