As a teenager I was told to drink lemon juice in hot water every morning to counteract a greasy, spotty skin. It didn't have a noticeable effect on my skin, but I was fairly sure it did my tooth enamel a power of no good...
However a lemon is a low carb fruit, so I can't imagine a glass of unsweetened lemon juice would do much at all to
raise glucose levels, so depending on your meter you may see a slight rise, no rise or maybe a slight decrease.
I'd tend to be more concerned with keeping high sugar and starchy foods out of my diet, as avoiding these will have a far greater impact on reducing glucose levels a little drink of lemon.
Robbity