Fixes #8444.

Why

• opentelemetry-proto#782 now recommends that OTLP exporters enforce configurable outbound request/message size limits.
• OpenTelemetry Java already limits response body reads, but OTLP exporters did not enforce outbound request/message limits.

What

• add configurable request/message size limits to OTLP HTTP and gRPC exporter builders with a default of 64 MiB
• enforce the limit in the shared OTLP HTTP and gRPC exporter layers before dispatching requests
• cover both known-size binary payloads and unknown-size payloads (for example OTLP HTTP JSON) by measuring when needed
• add focused tests for over-limit failure behavior and builder configuration plumbing
• update docs/apidiffs/current_vs_latest/opentelemetry-exporter-otlp.txt for the new public builder methods

Testing

I was not able to complete local Gradle verification on this Windows machine because Gradle / the Kotlin compiler daemon repeatedly failed to start due to paging-file / metaspace limits before compilation finished. The changes are pushed for CI verification.