Use *Data proto messages for file exporter (#3809)

Fixes #2390

## Changes

The `*Request` proto messages are design for RPC/REST communication
(request, response style) and may contain in the future information
specific to this type of communication.

The `*Data` proto messages are design for cases where only the telemetry
data must be encoded without ony OTLP specific metadata.

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

CHANGELOG.md

@@ -54,6 +54,8 @@ release.
 
 ### OpenTelemetry Protocol
 
+- Use `TracesData`, `MetricsData` and `LogsData` proto messages for file exporter.
+
 ### Compatibility
 
 ### SDK Configuration

specification/protocol/file-exporter.md

@@ -48,7 +48,7 @@ This defines the first version of the serialization scheme.
 The data must be encoded according to the format specified in the
 [OTLP JSON Encoding](https://github.com/open-telemetry/opentelemetry-proto/blob/main/docs/specification.md#json-protobuf-encoding).
 
-Only top-level objects, `ExportTraceServiceRequest`, `ExportMetricsServiceRequest`, and `ExportLogsServiceRequest` are supported.
+Only top-level objects, `TracesData`, `MetricsData`, and `LogsData` are supported.
 
 Files must contain exactly one type of data: traces, metrics, or logs.