The Metrics tool (M), is present in the Toolbar.
It activates the metrics mode of the Glyph window where you can edit glyph sidebearings and advance width. If there is no open Glyph window, clicking the “Metrics tool” opens a new Glyph window with the currently selected glyph(s) in it, and activates the metrics mode. This behavior is the same as when Window > New Metrics Window (or New Metrics Tab) menu item is selected. These commands exist in FontLab mostly for backward compatibility with FontLab Studio 5.
If you have no open Glyph window but instead are in a Font window, typing M will select the glyph for the Latin letter “m” rather than activating the Metrics tool.