samplingprocessor: batch all spans together for downstream processors

[chris-smith-zocdoc]

Is your feature request related to a problem? Please describe.
The current implementation of samplingprocessor calls nextConsumer.ConsumeTraces once per received batch of spans. Spans arriving from different services would necessarily be in separate batches. I'd like nextConsumer.ConsumeTraces to be invoked once, with all the spans for a given trace. This would allow downstream processors to operate on the entire trace.

Describe the solution you'd like
The samplingprocessor already has all spans available, the change would be minimal, instead of invoking ConsumeTraces in a loop here https://github.com/open-telemetry/opentelemetry-collector/blob/master/processor/samplingprocessor/tailsamplingprocessor/processor.go#L183-L185 the processor would create a new pdata.Traces and add all spans to it.

Conceptually this is similar to how the groupbytrace processor works.

If this is ok, I can submit a pr for it.

[chris-smith-zocdoc]

Fixed in #1864