chore: prep v0.3.0 release

[mabdinur]

What does this PR do?

Prepares the codebase for v0.3.0 release by updating version numbers and adding documentation for Metrics and Logs API support.

Motivation

This release includes significant new features:

• OpenTelemetry Metrics API support (feat(otel): metrics support #127)
• OpenTelemetry Logs API support (feat(logs): add support for otlp logs export #144)
• Sync export mode for trace exporter (feat(exporter): add sync export mode #149)
• Enhanced config reporting (feat(config_visibility): enhanced config reporting [APMAPI-1693] #126)

Version references and documentation need to be updated to reflect the new release.

Additional Notes

• Changelog entry documents all changes since 0.2.1
• README now includes Metrics and Logs sections with links to Datadog documentation
• After merge, create and push tag datadog-opentelemetry-v0.3.0 to trigger automated publish

[mabdinur]

Closing in favor of: #173