Improve histogram images in metrics data-model.md

There are a handful of helpful images in the histogram readme that do a fantastic job of demonstrating how histograms work. Unfortunately, there are a couple of problems with these:

1. The text aliasing looks awful / low quality text.
2. The background is transparent, which means that the images are hard/impossible to see all the detail in dark mode.

I think it would be great if we could put a background color in place (to help both light and dark mode readers) and to maybe recreate these images as line art (svg) so the text is readable.

@jsuereth did you create these originally? If not, do you know who? I was thinking that starting with the original might be simpler than trying to completely recreate these.

Images in question live here: https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/metrics/img

• model-layers-stream.png
• model-delta-sum.png
• model-cumulative-sum.png
• model-gauge.png
• model-delta-histogram.png