This drug blocks glucose production in the liver, as well as increases glucose uptake by the cells of the body. These two effects yield a lower blood glucose. Metformin can also increase your blood lactate level, irrespective of exercise stress or oxygen availability. Lactate builds up due to Metformin blocking glucose production in the liver, as well as its ability to block oxidative respiration (aerobic respiration) in the mitochondria. Lactate, which is just buffered lactic acid, is an excellent fuel source for muscles, among other organs. Lactic acidosis from Metformin, however, is uncommon and unpredictable, and is generally associated with more serious medical problems. Nevertheless, maybe some extra lactate helped you? (pure speculation here), or more likely, perhaps it simply encouraged more blood sugar to enter your cells (you became less insulin resistant)
HR monitoring is probably not an accurate way to determine effort, or your body’s aerobic efficiency per work load, or power output, when on a beta-blocker. It’s honestly not even a super reliable marker when one’s not on beta-blockers. In general beta-blockers will blunt your VO2max by blunting your max HR as well as your bodies other pro-performance responses to adrenaline. The other issue is that maybe the metoprolol prevented you from going in and out of afib during exercise, which is often asymptomatic, thereby increasing your stroke volume and efficiency in delivering oxygen to your muscles. Some new heart rate wrist watches, like the apple watch, can actually alert you to certain arrhythmias occurring. Some doctors are recommending them because they are cheaper than other more expensive, medical-grade, arrhythmia monitoring devices.
Anyway–they both seemed to improve your performance, so good news regardless.