Remove decode_message span

[balegas]

Remove decode_message span and store decode execution time into parent span. so we later can find outliers.

We currently create a decode_message span for every transaction we handle, which is generating loads of spans

[kevin-dp]

I'm wondering whether we want to add this as span attributes or events.
Here's an explanation on when to use attributes vs events: https://opentelemetry.io/docs/concepts/signals/traces/#when-to-use-span-events-versus-span-attributes

It would make sense to me to add an event at the start of decoding the message and an event at the end of the decoding and from those timestamps we could compute the duration. Or we could just add it as an attribute as suggested in this issue. Not sure what's best.

[balegas]

Sounds good to me, but cc.ing @KyleAMathews who has more experience with OTEL.

[KyleAMathews]

Events are useful if we want to record info specifically about the event e.g. normal log stuff like errors w/ stack traces or "this odd thing happened that we want to make a note of" etc.

add an event at the start of decoding the message and an event at the end of the decoding and from those timestamps we could compute the duration

This definitely should be an attribute as it's not trivial (or perhaps possible in this case — I'd have to investigate) to calculate stuff across events like this. It's much easier/neater to just have the code do the computation and put it in as an attribute.