Spanmetrics connector to export e2e trace latency

[jayaganesh1997]

Currently the spanmetrics connector only exports span level metrics.
It would really help to have the e2e trace latency exported to measure the e2e time for asynchronous process (for sync process, the root span duration is the e2e duration).

Technical challenge: trace is an span append only operation and hence it is not practical to define the end of a trace and thereby measure the e2e time.
Possible solutions:

1. similar to groupbytrace processor where we can define the wait duration for collecting all the spans related to a trace to compute the e2e latency
2. automatically identify if the spanmetrics connector is used after the groupbytrace processor thereby making it easy to trivially export (by just keeping track of the global_min of the start_time and global_max of the end_time in the aggregateMetrics method) the e2e trace latency
3. since tailsampling processor already has e2e trace latency based sampling, it would be easy to just export the latency value when the trace gets sampled (although at the code level this might need significant changes and doesn't adhere to the metrics export for the rest of the configurations)