Display number of measurements per row

[huddlej]

Description

During a demo of the measurements panel to one of the CCs, a user asked if there was a way to know how many measurements were being shown in each row of a panel or overall (see original context from Slack).

We should consider how we want to display the number of measurements shown whether as:

1. the total measurements shown across all rows
2. the total of measurements per row
3. the measurements per coloring group within each row

A simple implementation could be the first option which would mirror how we display the number of genomes shown at the top of the main Auspice panel (e.g., "Showing 401 of 401 genomes sampled between Jan 1968 and Nov 2011.").

For the second option, we could show the number of measurements like "(N=100 measurements)" as an additional row in the y-axis label for each grouping.

For the third option, we could show the number of measurements per coloring in the tooltip for the mean/std dev display.