Metacam is a very good and usually safe Non steroidal anti-inflammatory painkiller. It is prescription only and many vets use similar generic products like Loxicom or Meloxidyl.
The most common side effects are vomiting, diarrhoea and loss of appetite.
The debate is often when to start using it in older dogs and a visit to your vet for a clinical exam would help with deciding when medication is needed. If there is marked loss of range of motion and pain on manipulation it may well help to start sooner and preserve mobility and prevent muscle wastage etc.
Lots of people use Glucosamine/Chondroitin supplements like Synoquin or Seraquin as a starting point and add in NSAIDs when the lameness deteriorates. The clinical evidence for these is mixed but lots of owners report an improvement in mobility once they have been taken for long enough.
Difficult to say from the photo but weight loss is actually one of the very best ways of helping esp if the dog is overweight and many chocolate Labs are. Losing weight is free and will make a great difference in the long term.
Another option is a diet like Hills J/D which is again ideal to start before they get really bad.