Support Opentelemetry 0.31

[asonix]

Updates to 0.31

• set_parent now returns a result, it only fails if the span is disabled or the span has "already started". I think neither of these is the case for tracing-actix-web's use
• examples are updated
• readme is updated
• version is bumped

[batleforc]

Hi @asonix,
is it ready ? I was waiting for this feature to come back to this lib has it's the best version I've found ?

[AzHicham]

Same here :)

[vriesk]

@LukeMathWalker could we have a release with this?