I have been very careful with the string trimmer. I think the damage is from the original city planting or freeze damage. Should I use a chisel and cleanup the wounds or just let it be? It is a Maple and the presence of the insect in the photo is alarming as well. The tree us doing well.
Not everyone treats trees with care. This looks like the kind of damage that would happen if you had a potted or balled and burlapped tree and let it bounce on the gate of a pickup truck once or twice. Or this could be a lawnmower butting against the trunk two or three times.
You might think this is trivial but trees deal with wounds differently than animals and people. Trees compartmentalize damage. They grow callous at the edge of the wound and the edges gradually close up over time.
For this tree at least one third of circumference is no longer transporting nutrients up to the top growth. You should expect to see slower growth on that side. Bridge grafting is a possibility.
The least effort solution is to remove the grass up to three feet (1 M) away from the trunk and mulch or top dress annually. This will reduce competition for nutrients and water and reduce the possibility of further trimmer damage.
Do not take a chisel or other tools to the bark. "Leave her be" is the best.
While you are careful, not all workers on your yard may be equally so.
You may want to put in some form of trunk protector. Chunks of weeping tile will work. Bottom edge should be under the mulch. Should have a ring of mouse proof holes above the mulch for ventilation, and should not be in general contact with the bark of the tree.
A ring of stakes tall enough to prevent the line from hitting the trunk also works. Most weed eaters do about a 14" circle, so having stakes about 7 inches apart, about 4 inches from the trunk would be effective. 16" chunks of scrap re-bar work well.