Handle partial success responses from OTLP export services

[joaopgrassi]

Problem Statement

Following the open-telemetry/opentelemetry-specification#2696, each OTel SDK is encouraged to handle the resulting error message string in an appropriate way, considering existing norms. This requires OTLP v0.19. If I got it correct, this PR already added support for it in pdata #6361

Proposed Solution

For each of the Export*ServiceResponse methods used by OTLP Trace and Metrics exporters, construct an error and call the appropriate handler with the error message string and the number of spans/points(/logs) dropped. Future OTel specifications may call for optional treatment of the number of dropped items, but presently that is just additional information to include in the handled error.

For context, here's a few SDKs that already implemented handling partial success responses: Go, PHP

cc @jmacd @tigrannajaryan @evan-bradley

[evan-bradley]

I'll work on implementing this.

[jmacd]

I would say that ideally the return from a PartialSuccess would factor into the counts of dropped points/spans/logs vs accepted points/spans/logs. Was this considered? The way the PRs #6971 and #6970 were written, these become total errors and the result is exporterhelper counts them as such, meaning:

1. The metrics are wrong in a way that PartialSuccess was meant to preserve.
2. The producer of the metrics doesn't get propagated results telling them how many of their points dropped.

I think the collector's metrics should factor the meaning of PartialSuccess in and also propagate the partial success to the original producer. Can we leave this open, hoping for improvements?