Add OpenTelemetry tracing

See https://opensource.posit.co/blog/2026-05-07_opentelemetry/. This is very simple to implement - it just needs some variables setting in `.Renviron` - and could provide a lot of useful info on number of users, size of datasets and any crashes. Unlike google analytics, the data doesn't appear to be PII so could be used on all users. I discussed briefly with Alex and Nicola but it probably needs someone to confirm it is not PII before implementing.