stats: add optional gzip compression to OTLP/HTTP stats sink

[prashanthjos]

Adds a field (NONE default, GZIP) to the OpenTelemetry sink that gzip-compresses
the OTLP/HTTP export body and sets (~14x smaller payloads). Defaults to
NONE for backward compatibility; HTTP path only. Includes unit tests for both paths.

Commit Message: stats: add optional gzip compression to OTLP/HTTP stats sink
Additional Description: Adds a compression enum field (NONE default, GZIP) to the OpenTelemetry stat sink's SinkConfig. When set to GZIP on the http_service (OTLP/HTTP) export path, the serialized ExportMetricsServiceRequest payload is gzip-compressed and Content-Encoding: gzip is set on the outbound request, reducing export bandwidth (~14x smaller on a representative 500-metric batch: 98,178 -> 6,800 bytes). Reuses the existing gzip compressor extension rather than hand-rolling zlib. Only honored for the HTTP path; the gRPC path continues to configure compression via the GrpcService. The OTLP/HTTP collector must support gzip decoding, per the OTLP/HTTP specification.
Risk Level: Low
Testing: Unit tests added covering the uncompressed path (no Content-Encoding, raw body) and the gzip path (Content-Encoding: gzip, gzip magic bytes, round-trip decompresses to the original proto). Existing sink tests pass.
Docs Changes: Proto field documentation (SinkConfig.compression + Compression enum).
Release Notes: Added a changelog fragment under changelogs/current/new_features/.
Platform Specific Features: None
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @markdroth
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #46246 was opened by prashanthjos.

see: more, trace.