metronome marks aren't written to lilypond files

[drammock]

music21 version

9.9.1

Operating System(s) checked
Linux

Problem summary

MetronomeMark is not included when converting Stream object to Lilypond score.

Steps to reproduce

import music21 as mm

keysig = mm.key.Key("a-")
tempo = mm.tempo.MetronomeMark(number=87, referent=mm.note.Note(type="quarter"))
timesig = mm.meter.TimeSignature("3/4")
melody = [
    mm.note.Note(pitch=pitch, duration=mm.duration.Duration(type="quarter"))
    for pitch in keysig.getPitches()
]
stream = mm.stream.Stream([keysig, tempo, timesig, *melody])
stream.write("lily", "path/to/output/file.ly")

Expected vs. actual behavior

actual output (note the lack of any \tempo quarter = 87 or similar):

\version "2.24" 
\include "lilypond-book-preamble.ly"
    
color = #(define-music-function (parser location color) (string?) #{
        \once \override NoteHead.color = #(x11-color color)
        \once \override Stem.color = #(x11-color color)
        \once \override Rest.color = #(x11-color color)
        \once \override Beam.color = #(x11-color color)
     #})
    
\header { } 
\score  { 
 \new Voice { \new Voice { \key aes \minor 
                \time 3/4
                aes' 4  
                bes' 4  
                ces'' 4  
                des'' 4  
                ees'' 4  
                fes'' 4  
                ges'' 4  
                aes'' 4  
                 } 
               
 
           } 
         
 
  } 
 
\paper { }
\layout {
  \context {
    \RemoveEmptyStaves
    \override VerticalAxisGroup.remove-first = ##t
  }
 }

More information

My particular use case was a bit more complicated (instantiating a converter, extracting just the LyScoreBlock from its context, and writing a whole series of those one-line scores to a single page, with a custom header for the .ly file). FWIW, what I hacked together as a workaround is:

def insert_metronome_mark_into_score(score, tempo):
    tempo_range = mm.lily.lilyObjects.LyTempoRange(tempo)
    steno = mm.lily.lilyObjects.LyStenoDuration("4")  # "quarter" doesn't work
    tempo_mark = mm.lily.lilyObjects.LyTempoEvent(
        tempoRange=tempo_range, stenoDuration=steno
    )
    score[
        0
    ].scoreBody.music.compositeMusic.contents.music.compositeMusic.contents.sequentialMusic.musicList.contents[
        0
    ].music.compositeMusic.contents.sequentialMusic.musicList.contents.insert(
        2, tempo_mark
    )

I mention it because inserting the more simple mm.lilyObjects.LyTempoEvent(scalar="87") fails (lilypond seems to choke on \tempo 87 with no steno and equals sign). Also fails if the steno is "quarter" instead of "4" --- the docstring example code uses "quarter" so may want to change that.