Note to Much Ado About Nothing, 4.1.156: "I have only been silent so long / And given way unto this course of fortune, / By noting of the lady"
I have only been silent so long / And given way unto this course of fortune, / By noting of the lady: I have been silent so long, and not interfered with these happenstances, only because I have concentrated on observing the lady.
I have rendered "course of fortune" as "these happenstances" because I believe that Friar Laurence's phrase indicates he believes that what has just happened is all a matter of fortune, i.e., bad luck. His following lines make clear that his observations of Hero have convinced him that Hero is not guilty of anything.