feat(OTLP): add zstd and gzip compression support to HTTP OTLP exporter

[scottgerring]

Fixes #
#3083

Changes

Straightforward enough! None of our clients support this natively, so we have to do the compressing ourselves. Have copied the pattern used by the gRPC exporter in terms of features and API, and moved a bit of code around so we can share it.

Test coverage added, and have also fired up the OTLP example and pointed it at a collector with both compression formats.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 81.65468% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.5%. Comparing base (42139cb) to head (52938a4).

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/http/mod.rs	88.9%	26 Missing ⚠️
opentelemetry-otlp/src/exporter/http/metrics.rs	0.0%	10 Missing ⚠️
opentelemetry-otlp/src/exporter/http/trace.rs	0.0%	8 Missing ⚠️
opentelemetry-otlp/src/exporter/http/logs.rs	0.0%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3092     +/-   ##
=======================================
+ Coverage   80.1%   80.5%   +0.4%     
=======================================
  Files        126     126             
  Lines      21957   22198    +241     
=======================================
+ Hits       17603   17890    +287     
+ Misses      4354    4308     -46     

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[cijothomas]

could this be detected and fail-fast at exporter build time itself?

[Copilot]

Pull Request Overview

This PR adds HTTP compression support (gzip and zstd) to the OTLP HTTP exporter, aligning it with the existing gRPC exporter capabilities. It implements compression for all signal types (traces, metrics, logs) and includes comprehensive test coverage.

• Adds optional gzip and zstd compression support via feature flags (gzip-http, zstd-http)
• Refactors compression resolution logic to be shared between HTTP and gRPC exporters
• Updates all HTTP export methods to handle compressed payloads with appropriate headers

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
opentelemetry-otlp/src/lib.rs	Updates imports to conditionally include WithTonicConfig
opentelemetry-otlp/src/exporter/tonic/mod.rs	Refactors to use shared compression resolution logic
opentelemetry-otlp/src/exporter/mod.rs	Adds shared compression resolution function
opentelemetry-otlp/src/exporter/http/trace.rs	Updates trace exporter to handle compression headers
opentelemetry-otlp/src/exporter/http/mod.rs	Core HTTP compression implementation and test coverage
opentelemetry-otlp/src/exporter/http/metrics.rs	Updates metrics exporter for compression support
opentelemetry-otlp/src/exporter/http/logs.rs	Updates logs exporter for compression support
opentelemetry-otlp/examples/basic-otlp-http/src/main.rs	Example demonstrating compression usage
opentelemetry-otlp/examples/basic-otlp-http/Cargo.toml	Adds compression feature flags to example
opentelemetry-otlp/Cargo.toml	Adds compression dependencies and feature flags

Comments suppressed due to low confidence (1)

opentelemetry-otlp/src/exporter/http/mod.rs:295

• The variable name Compression conflicts with the type crate::Compression. Consider using flate2::Compression or aliasing to avoid confusion.

                let mut encoder = GzEncoder::new(Vec::new(), Compression::default());

[cijothomas]

good catch! Wondering how our CI didn't catch this before!

[cijothomas]

nit: Move Content-Encoding to const string, or is it available from http crate already?

[cijothomas]

Suggested change

	/// The compression algorithm to use when communicating with the collector.
	/// The compression algorithm to use when communicating with the OTLP endpoint.

[cijothomas]

nit: The variable gives the wrong impression that compressed_body is always compressed. But if Compression is None, then its just returning existing body as-is.
May be:
if (self.compression is None)
{
Ok(body,content_type,None)
}
else
{
call self.compress_body(...),
}

[cijothomas]

examples are currently kept basic.-showing absolute minimum required to work. I suggest to keep it that way in this PR. If we make a separate decision about improving these examples to show more scenarios, we can do it for this+ the gRPC examples too, in a separate PR.

[cijothomas]

https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-otlp/src/lib.rs#L269-L271 The kitchensink would be a good place to showcase this.

[cijothomas]

Looks good! Requesting changes just for the below:

1. See if we can detect invalid/incorrect compression setting at build_exporter time itself and not in actual export. Eg; user enabled compression via env_variable, but the feature-flag is not enabled.
2. Add a changelog as this is a new capability
3. Lets keep example as-is in this PR.