Update ExportResult: use std::result::Result type

[frigus02]

Update ExportResult to match the latest version in the spec:

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#exportbatch

ExportResult is one of:

• Success - The batch has been successfully exported. For protocol exporters this typically means that the data is sent over the wire and delivered to the destination server.
• Failure - exporting failed. The batch must be dropped. For example, this can happen when the batch contains bad data and cannot be serialized.

Since this only defines 2 states now (error and success), we should probably go the idiomatic Rust route and use a std::result::Result.

[jtescher]

Is this type actually necessary anymore if there are only two possibilities? or would a result type of some kind be more idiomatic?

[TommyCpp]

I think we can define a ExportError as Failure and use Result as return type.

[frigus02]

Yeah, that's a good point. Let's use a Result type. I guess we either wait until #286 and make some kind of ExportError or we start with Box<dyn Error + Send + Sync + 'static>?

I wonder if we should keep the name ExportResult as a type alias or if we just go with a standard Result.

[jtescher]

Seems like the only purpose here may be calling an error handler as both success and failure will drop the span data. If that is the case then #263 may be the only code path that would use this type?

[frigus02]

Yeah, that's how I understand this as well.

In theory someone could implement a custom span processor to control when spans are exported. This would mean they would also be exposed to the export result.

I'd say something like this should be good enough:

type ExportResult = Result<(), Box<dyn std::error::Error + Send + Sync + 'static>>