chore: various updates to improve debugging apps

[Fraser999]

This PR comprises several changes:

1. disables connection pooling in OAuth job to avoid stale connection errors, and reduces log level from error to warn for failing to refresh the OAuth token (as this is a non-critical error I believe)
2. avoids recording the current span twice (it's recorded as the last entry in "spans" since we enable with_span_list)
3. supports including file and line number in log lines by setting the env var TRACING_WITH_FILE_AND_LINE_NO to a non-empty value
4. a somewhat larger change to env var handling:
   • the existing init4 function is now deprecated in favour of a similar init function which requires the tracing and metrics configs to be passed in. The idea is that callers will add these configs to their main config of their app, and hence all the env vars including the tracing and metrics ones will be visible via the inventory and checkable via the check_inventory methods
   • added the tracing config, which internally holds an optional OTEL config
   • updated check_inventory's error case to also include env vars which are set to an empty value
   • added a set of types like OptionalBoolWithDefault which are designed to be used as members of config structs. These are provided with a const generic arg to be used as the default, allowing the config to not have to deal with Option<T> where the optional value is always set to Some after calling from_env.

[Fraser999]

• chore: various updates to improve debugging apps #123 👈 (View in Graphite)
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