[FEATURE] Make it clear that metric_prefix is a list of atoms, or handle more gracefully

[DaniruKun]

Is your feature request related to a problem? Please describe.
From the docs about PromEx.Plugins.Ecto it is unclear what is the type of the metric_prefix argument. The naive assumption is that it is a binary (or String), but looking at the code, it assumes a list of atoms is passed:

prom_ex/lib/prom_ex/plugins/ecto.ex

Line 92 in d769563

metric_prefix ++ [:repo, :init, :status, :info],

This is unclear behaviour that isn't documented or handled gracefully:

    ** (EXIT) an exception was raised:
        ** (ArgumentError) argument error
            :erlang.++("someapp_ecto", [:repo, :init, :status, :info])
            (prom_ex 1.7.1) lib/prom_ex/plugins/ecto.ex:92: PromEx.Plugins.Ecto.init_metrics/1
            (prom_ex 1.7.1) lib/prom_ex/plugins/ecto.ex:70: PromEx.Plugins.Ecto.event_metrics/1
            (prom_ex 1.7.1) lib/prom_ex.ex:604: PromEx.init_plugin/3

Describe the solution you would like to see

1. Make it clear in the documentation what type is expected + a short example.2.
2. Ideally, handle both types (e.g. if a snake_case prefix is supplied as a string, convert it to a list of atoms).

How would you expect this feature to work
A description of a possible API, behaviour, modules, etc.

Additional context
Add any other context or screenshots about the feature request here.