[RELEASE] Release opentelemetry-cpp v1.27.0

[marcalff]

Released 2026-05-13.

Must have:

• [EXPORTER] OTLP HTTP exporter reads unbounded response #3958

Desirable:

• [OTLP GRPC] Allow user use custom grpc::ChannelArguments #3990
• [CI] update code.coverage job to report on all components and preview features #4002
• [EXPORTER] Implement non-utf8 string to bytes in OTLP #3991
• [SDK] env var durations non corforming to spec #4019
• Base2ExponentialHistogramAggregation::Merge() silently drops bucket counts when merged index range crosses zero (count > sum_of_buckets) #4068

Postponed:

• [Code health] clang-tidy cleanup #2053
• [CMAKE] Upgrade third-party dependencies #4018
• [CODE HEALTH] Fix clang-tidy bugprone-exception-escape warnings in API #3964

[dbarker]

Desirable:

• [CMAKE] Upgrade third-party dependencies #4018

[dbarker]

Desirable:

• [CODE HEALTH] Fix clang-tidy bugprone-exception-escape warnings in API #3964

[dbarker]

Desireable:

• Base2ExponentialHistogramAggregation::Merge() silently drops bucket counts when merged index range crosses zero (count > sum_of_buckets) #4068