`metronome' provides a simple metronome for Emacs.  Note that, as I
understand it, no asynchronous pure-Elisp metronome can have
extreme accuracy, due to the nature of Emacs timers, but a
foreground, synchronous, metronome is good enough for my purpose.
The background metronome may hiccup occasionally if you have Emacs
doing other things while it's running.

(My need is simply to get a quick tempo check, lasting a few
seconds, before I start playing; I don't play along with
metronomes.)

There is also a "reverse metronome" (`metronome-compute') that
computes bpm from keyboard taps.

The metronome works in beats per minute (bpm), but also knows a
selection of traditional music tempo indications (like largo and
allegro).