[…] nor circumscrib'd alone Their growing virtues, but their crimes confin'd; Forbade to wade through slaughter to a throne, And shut the gates of mercy on mankind,
The struggling pangs of conscious truth to hide, To quench the blushes of ingenuous shame, Or heap the shrine of Luxury and Pride With incense kindled at the Muse's flame.
Again, we have to combine two stanzas because the sentence continues across the stanza break—more enjambment!
Not only does the poverty of the villagers prevent ("circumscribe" = prevent) them from developing the virtues that would get them remembered in the history books, it also keeps them from committing crimes.
Here are some examples of the crimes these poor villagers just don't have time to commit, since they're busy working to put food on the table:
They don't have time to wade through blood and gore to kill a king on his throne, or to act all merciless to people.
Another metaphor there! Slamming the "gates of mercy" is a metaphor for being merciless. (Try to work that one into everyday conversation. You can tell your athlete friends to "shut the gates of mercy" on the other team!)
The villager's lot in life keeps them from trying to hide the truth, especially when the truth is struggling and conscious of BEING the truth.
Their situation likewise keeps them from trying to hide their blushes. After all, a blush indicates that you're ashamed of something, right? So if you hide your blushes, you're hiding your true feelings. So this one goes along with the previous line.
"Ingenuous" means innocent.
And there's more metaphor here. You know how when you blush, your face feels hot? We talk about "quenching" flame, so here, the blush is the metaphorical flame that's getting "quenched."
The poor villagers also don't have the chance to use fancy and flattering words to build a metaphorical shrine to the personified Luxury and Pride.
The Muses were the goddesses in Greek and Roman mythology who were responsible for inspiring artists, musicians, and poets. So the "incense" that was lit at the Muse's flame must be a pen that is metaphorically kindled, lit up, or inspired by the Muses.