fix(sdk): resolve exporter deadlock on constrained tokio runtimes

[bryantbiggs]

Summary

• Add BlockingStrategy that captures the tokio runtime handle at construction and enters the runtime context via Handle::enter() before calling futures_executor::block_on() on dedicated background threads
• Update BatchSpanProcessor, BatchLogProcessor, and PeriodicReader to use BlockingStrategy instead of bare futures_executor::block_on()
• Falls back to plain futures_executor::block_on() when no tokio runtime is available

Problem

The default thread-based processors call futures_executor::block_on(exporter.export(...)) on their dedicated worker threads. When the exporter uses tonic/gRPC, the export future depends on tokio tasks (e.g. tonic's Buffer worker spawned via tokio::spawn) that can only be polled by tokio worker threads. If all tokio worker threads are blocked — single-threaded runtime (current_thread), or multi_thread with 1 worker (common in 1-vCPU k8s pods) — this creates a circular wait: the worker thread waits for the export to complete, but the export can't complete because no tokio thread is available to poll the Buffer worker.

Reproduction and detailed analysis in #3356 (comment): #3356 (comment)

Minimal repro gist: https://gist.github.com/bryantbiggs/62737e105525fe341090d0ad97de2178

Scenario	force_flush()	shutdown()
current_thread	Hangs forever	Returns Err(Timeout(5s)), worker thread stays stuck
multi_thread(1) + tokio::spawn	Hangs forever (entire runtime freezes)	Same pattern
multi_thread(default workers)	Returns immediately	Returns immediately

Solution

BlockingStrategy::new() is called during processor construction (while inside the tokio runtime context). It calls Handle::try_current() to capture the runtime handle. On the dedicated background thread, blocking_strategy.block_on(future) enters the runtime context via Handle::enter() before calling futures_executor::block_on(). This makes tokio types (spawn, timers, IO) available without taking ownership of the reactor — IO continues to be driven by the runtime's own threads.

When no tokio runtime is available (e.g., non-tokio environments), it falls back to plain futures_executor::block_on() — preserving existing behavior.

Scope

This is the scoped-down version of #3356, containing only the bug fix as suggested by @scottgerring in #3356 (comment). The experimental async runtime removal is intentionally excluded and can be discussed separately.

Fixes #2802

Test plan

• cargo check -p opentelemetry_sdk --all-features
• cargo check -p opentelemetry_sdk --no-default-features
• cargo clippy -p opentelemetry_sdk --all-features -- -Dwarnings
• cargo test -p opentelemetry_sdk --features="testing" — 295 passed, 0 failed, 3 ignored (pre-existing)
• cargo check -p opentelemetry-otlp --all-features

[codecov]

Codecov Report

❌ Patch coverage is 89.51613% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.3%. Comparing base (9650783) to head (f9608c4).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/periodic_reader.rs	82.3%	6 Missing ⚠️
opentelemetry-sdk/src/logs/batch_log_processor.rs	88.8%	4 Missing ⚠️
opentelemetry-sdk/src/trace/span_processor.rs	92.6%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #3380    +/-   ##
======================================
  Coverage   83.2%   83.3%            
======================================
  Files        128     128            
  Lines      25045   25164   +119     
======================================
+ Hits       20858   20965   +107     
- Misses      4187    4199    +12     

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