Add % var explained in trajectory plots for q2

This may be a bit difficult because we would like to have the percent var. explained on the axis for the volatility plot -- while still specifying it in the command.

For example in the tutorial we have:

```bash
qiime longitudinal volatility \
...
--p-default-metric PC1 \
...
```

but it would be difficult to have this as "PC1 (#%)" so default % is removed. This may be helpful to build intro the trajectory types that could be a mix between Metadata and Ordination types.

Start with the qiime longitudinal volatility command and the tutorial example using --p-default-metric PC1; identify where the trajectory plot axis label is assembled. Done means the volatility plot can show the selected metric with its explained-variance percentage while the metric remains specified through the command option.

Written by the indexing model from the issue text.