Set open telemetry levels via an environment variable

[Modularius]

Summary of changes

Changes the way we set OpenTelemetry levels and tidies up some spans.

• Removed otel_level cli option from all components.
• Modified OpenTelemetry filter layer to use environment variable "OTEL_LOG" to determine which level to filter crates/modules on.
• Removed references to the otel target and target = "otel" syntax in spans/events (This accounts for most files changed). This target is not really useful, though OpenTelemetry-only targets may be introduced in the future.
• In trace-to-events, functions processing individual channels moved to their own file channels.rs. This allows the observability level for these to be set via including trace_to_events::channels=???? in OTEL_LOG. At present use info to include channels, and warn/error/off to exclude.
• Level of spanned_root_as functions changed to trace from info.
• Added digitiser_id tag to Digitiser Event List span in digitiser aggregator.

Instruction for review/testing

• General code review
• Has been tested on simulated data