opentelemetry-exporter-otlp-proto-http: add max request size limit to OTLP HTTP exporters

[Krishnachaitanyakc]

Description

Implements the client side of opentelemetry-proto#782: the OTLP/HTTP exporters drop a serialized request larger than a configurable limit (measured before compression) instead of sending it. Adds a max_request_size argument and OTEL_EXPORTER_OTLP_MAX_REQUEST_SIZE (plus per-signal _TRACES_/_METRICS_/_LOGS_ overrides) to the trace, log, and metric HTTP exporters. Default 64 MiB (the spec's RECOMMENDED value, enabled); 0 (or any non-positive value) disables. Mirrors opentelemetry-go#8157.

Behavior-change note: with the default-on limit, batches whose serialized size exceeds 64 MiB (previously sent) are now dropped client-side and recorded as a failed export. gRPC exporters are out of scope here and will follow separately.

Related to #4533 (this is the proactive request-size half; reactive 413 splitting is tracked in #5032).

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• Unit tests across all three HTTP exporters: oversized request dropped before Session.post (asserted not called); 0/negative disables; env precedence (constructor > per-signal > generic > default) including malformed/empty fallback; failed-export metric records error.type.
• Discriminating "before compression" test: a payload whose gzip size is below the limit but uncompressed size exceeds it is still dropped.
• End-to-end test against a real local HTTP server: a normal payload is received; an oversized one never reaches the server.

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated