monitoring: use label_replace to fill empty fallback series with provided 'by' labels

Created by: Strum355

Previously, we had queries in the form of sum by (op)(increase(src_team_series_total{...}[5m])) OR on() vector(0). If there were gaps in the graph, this fills a zero'd series with no labels. This caused dashboard panels to add a spurious label as follows:

This PR changes the zero'ing generator to add labels to the zero'd series based on the by grouping with the value of None for each.

Before:

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After:

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