Yes, you can you use label replace to group all the misc together:

sum by (new_group) (
  label_replace(
    label_replace(my_metric, "new_group", "$1", "group", ".+"),
    "new_group", "misc", "group", "misc group.+"
  )
)

The inner label_replace copies all values from group into new_group, the outer overwrites those which match "misc group.+" with "misc", and we then sum by the "new_group" label. The reason for using a new label is the series would no longer be unique if we just overwrote the "group" label, and the sum wouldn't work.

I just wanted to add to the excellent answer by Alin Sînpălean. He splits the data into time series per day. You can similarly split them per month and then use the Grafana reduce "series to rows" transform to calculate the total per month. You need another sort transform to get the rows back in order. This only works well when you add a leading zero to the month number. Then you can use "{{year}}-{{zero}}{{month}}" as your legend which sorts correctly.

label_replace(
  up{job="prometheus"} + ignoring(year, month) group_right
    count_values without() ("year", year(timestamp(
      count_values without() ("month", month(timestamp(
        up{job="prometheus"}
      )))
    ))) * 0,
  "zero",
  "0",
  "month",
  "^[0-9]$"
)