1) Why a French morning all of a sudden? Matin is the same number of syllables. The iambs work the same either way. We are in Denmark, after all, not France.
2) Glow-worm? Is this an actual glow-worm? Or is this a metaphor? Is the moon a glow-worm? A star?
3) Isn’t a fading glow-worm an awfully cute harbinger of morning?
The glow-worm shows the matin to be near And ‘gins to pale his unaffected fire.