Enable reqwest and reqwest-blocking client creation with custom timeout

[lalitb]

Fix: towards #2576

Changes

This PR defers the creation of http_client until the configured timeout is accessible. Previously, http_client was initialized within HttpConfig::default(), where the timeout value was unavailable.

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

Attention: Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.

Project coverage is 79.3%. Comparing base (7c9447f) to head (ed14fff).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/http/mod.rs	0.0%	46 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2584     +/-   ##
=======================================
- Coverage   79.5%   79.3%   -0.2%     
=======================================
  Files        118     118             
  Lines      22476   22487     +11     
=======================================
- Hits       17874   17844     -30     
- Misses      4602    4643     +41     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cijothomas]

https://github.com/open-telemetry/opentelemetry-specification/blob/714b1ae1382ba57850f29ee94b854b2aa7d1be83/specification/protocol/exporter.md?plain=1#L67 should it be 10 instead?

[lalitb]

Good catch. It's indeed 10sec. Have update the changelog accordingly.

opentelemetry-rust/opentelemetry-otlp/src/exporter/mod.rs

Line 55 in dde68a0

pub const OTEL_EXPORTER_OTLP_TIMEOUT_DEFAULT: u64 = 10;

[cijothomas]

there is another issue about parsing env variables for timeout, I opened #2591 to track it separate from this PR

[cijothomas]

LGTM. For metrics we'd need a follow up as we need to use a per-request timeout override, to account for the the time taken for observable callbacks.

Also need to fix the changelog to state its 10 secs default, not 30.