consider paring back OTel UX code

[simonpcouch]

Currently, commons::commons(log = TRUE) sets some environmental variables and then subsets into ellmer:::otel_cache_tracer() to refresh the OTel tracing code.

This makes local tracing a bit more convenient (but is likely not CRAN-compliant). I'm not that worried about the local case and am fine to remove all of that in favor of noting in the onboarding flow that users should set them and restart R.

Without us doing this log trickery, on Connect, these variables will mostly be set appropriately automatically. That said, the user would still need to set OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT in a .Rprofile and then restart R (so that ellmer:::otel_cache_tracer() runs on ellmer package load) in order to deploy their app with tracing enabled. This is more cumbersome than I'd like—it doesn't seem like you should have to edit your .Rprofile and restart your R session to do this—but I don't see a way to make this 'just work' for users other than setting that envvar ourselves and then re-running ellmer:::otel_cache_tracer(). Ideally, ellmer could give us some entry point to refresh that setup.

(In the short term, we do still need to do this hacky subsetting for the OTEL_RESOURCE_ATTRIBUTES hotfix #66, but we can remove that by the time 0.1.0 is released.)

[simonpcouch]

On a related note, in order for these traces to flow, several things need to be true at the same time:

1. OpenTelemetry.Enabled and AllowContentInstrumentation. This defaults to false in Connect and requires an admin to enable it. (The latter is still internal.)
2. otel_enabled enables it for one content item. commons currently enables this Connect’s API with PATCH /__api__/v1/content/{guid} {"otel_enabled": true}
3. OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true tells ellmer to include message content. commons currently does this part for the user.

rsconnect could technically handle step 3, so an R restart isn't actually required:

Sys.setenv(OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT = "true")

rsconnect::deployApp(
  envVars = "OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT"
)

(There's also OpenTelemetry.ConversationCapture, which would prevent 'external export' if off but defaults to "full" in recent Connect versions.)

Regardless, if we really do need an admin involved for a relatively thorny setting enablement, 3. is probably not worth fretting about in comparison.