release: Release 4 gems

[github-actions]

This pull request prepares new gem releases for the following gems:

• opentelemetry-api 0.12.1 (was 0.12.0)
• opentelemetry-sdk 0.12.1 (was 0.12.0)
• opentelemetry-exporter-otlp 0.12.1 (was 0.12.0)
• opentelemetry-instrumentation-all 0.12.1 (was 0.12.0)

For each gem, this pull request modifies the gem version and provides an initial changelog entry based on conventional commit messages. You can edit these changes before merging, to release a different version or to alter the changelog text.

• To confirm this release, merge this pull request, ensuring the "release: pending" label is set. The release script will trigger automatically on merge.
• To abort this release, close this pull request without merging.

The generated changelog entries have been copied below:

---

opentelemetry-api

v0.12.1 / 2021-01-06

• FIXED: Eliminate warning about Random::DEFAULT on Ruby 3.0

---

opentelemetry-sdk

v0.12.1 / 2021-01-06

• FIXED: Fix several BatchSpanProcessor errors related to fork safety

---

opentelemetry-exporter-otlp

v0.12.1 / 2021-01-06

• (No significant changes)

---

opentelemetry-instrumentation-all

v0.12.1 / 2021-01-06

• (No significant changes)

[github-actions]

WARNING: An additional commit was added while this release PR was open.
You may need to add to the changelog, or close this PR and prepare a new one.

Commit link: b61ea96

Message:
fix: define default value for traceid ratio (#548)

[github-actions]

Release PR closed without merging.