feat: Add {otel} support

[schloerke]

Fixes: #4258

Not implementing #4265 - bindOtel(x), withOtel(expr, ..., bind = "all")
Fixes #4268 - options(shiny.otel.bind = "all") (and "none")
Fixes #4267 - ExtendedTask
Fixes #4282 - options(shiny.otel.bind = "session")
Fixes #4266 - options(shiny.otel.bind = "reactive_update")
Fixes #4268 - options(shiny.otel.bind = "reactivity") / options(shiny.otel.bind = "all")

---

News entry:

OpenTelemetry support (#4269)

• Added support for OpenTelemetry via {otel}. By default, if otel::is_tracing_enabled() returns TRUE, then {shiny} will record all OpenTelemetery spans. See {otelsdk}'s Collecting Telemetry Data for more details on configuring OpenTelemetry.

• Supported values for options(shiny.otel.bind) (or Sys.getenv("SHINY_OTEL_BIND")):

  • "none" - No Shiny OpenTelemetry tracing.
  • "session" - Adds session start/end spans.
  • "reactive_update" - Spans for any synchronous/asynchronous reactive update. (Includes "session" features).
  • "reactivity" - Spans for all reactive expressions. (Includes "reactive_update" features).
  • "all" [default] - All Shiny OpenTelemetry tracing. Currently equivalent to "reactivity".

• Spans are recorded for:

  • session_start: Wraps the calling of the server() function. Also contains HTTP request within the attributes.
  • session_end: Wraps the calling of the onSessionEnded() handlers.
  • reactive_update: Signals the start of when Shiny knows something is to be calculated. This span ends when there are no more reactive updates (promises or synchronous) to be calculated.
  • reactive, observe, output: Captures the calculation (including any async promise chains) of a reactive expression (reactive()), an observer (observe()), or an output render function (render*()).
  • reactive debounce, reactive throttle: Captures the calculation (including any async promise chains) of a debounce()d or throttle()d reactive expression.
  • ExtendedTask: Captures the calculation (including any async promise chains) of an ExtendedTask.

• OpenTelemetry Logs are recorded for:

  • Set reactiveVal <name> - When a reactiveVal() is set
  • Set reactiveValues <name>$<key> - When a reactiveValues() element is set
  • Fatal or unhandled errors - When an error occurs that causes the session to end, or when an unhandled error occurs in a reactive context. Contains the error within the attributes. To unsantize the error message being collected, set options(shiny.otel.sanitize.errors = FALSE).
  • Set ExtendedTask <name> <value> - When an ExtendedTask's respective reactive value (e.g., status, value, and error) is set.
  • <ExtendedTask name> add to queue - When an ExtendedTask is added to the task queue.

• All logs and spans contain the session.id attribute.

[jcheng5]

...after few remaining nits

[jcheng5]

Remove before merge

[schloerke]

I am going to merge with in it. And perform testing in a followup PR.