otel metrics support

One of these days we should take a swing at adding otel metrics to otel-cli. I don't know if it'll work out but it feels worth exploring.

e.g. (having not dug into metrics at all)

```sh
otel-cli counter foo.bar 1
otel-cli histo ???????
```

Should there be another layer of subcommands? e.g. `otel-cli trace span -n "my span"`? It feels heavy to me and I'm curious what others think.