[Geneva Exporter] Implement concurrent uploads and add metrics (open-telemetry#1653)

Refactor GenevaExporter to support concurrent batch uploads, to improve
throughput for high-volume telemetry - allowing multiple in-flight
upload requests per exporter instance (controlled by
max_concurrent_uploads).

It also introduces internal metrics to track the health and performance
of the export process and ensures proper Ack/Nack propagation. Also
added tests to ensure that the exporter correctly parses the user's YAML
configuration, ensuring all fields and authentication methods (like
certificates or managed identities) are deserialized properly.

Author: lalitb

rust/otap-dataflow/crates/otap/src/experimental/geneva_exporter/README.md

@@ -31,6 +31,12 @@ You should see `urn:otel:geneva:exporter` in the Exporters list.
 ./target/release/df_engine --pipeline config.yaml --num-cores 4
 ```
 
+### Notes on throughput knobs
+
+- `max_concurrent_uploads` limits how many batches the exporter will upload concurrently.
+- `max_buffer_size` is currently reserved for a future buffering/flush implementation.
+  It is accepted by config parsing but does not change runtime behavior yet.
+
 ## Test Configuration
 
 To test using the configuration file `otlp-geneva.yaml` provided
@@ -40,7 +46,7 @@ in this directory:
 # Start the collector
 ./target/release/df_engine \
   --pipeline crates/otap/src/experimental/geneva_exporter/\
-test-config-otlp-receiver.yaml \
+otlp-geneva.yaml \
   --num-cores 1
 
 # In another terminal, send test data: