Observability Support

[jterapin]

Following changes has been made to implement Observability:

Build Tools

• Add telemetry plugin to plugin list
• Update async_client_class and client_class files to include tracer when creating RequestContext

Core Changes

• Add Telemetry plugin, telemetry_provider config and Telemetry handler
• Add the following telemetry provider support: no-op and otel
• Add span wrappers for the following handlers: StubResponses, NetHttp and H2
• Update RequestContext to include a tracer attribute
• Add specs
• Add RBS support

Other Changes

• Add opentelemetry-sdk to the Gemfile (for testing purposes)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/version-3/CONTRIBUTING.md

Thank you for your contribution!

[alextwoods]

LGTM!