[FEATURE] Add a configuration to disable OpenTelemetry instrumentation

[B-Step62]

Problem Statement

Currently, there is no way to disable OpenTelemetry instrumentation. When we run the agent as a part of a system that already have OpenTelemetry enabled, Strands Agents always emits its spans even if we don't want them.

Proposed Solution

Add an environment variable to disable OpenTelemetry instrumentation.

I'm happy to contribute if it helps resolving this limitation faster.

Use Case

When we use Strands Agents as a part of existing system that already instrumented with OpenTelemetry and we don't want to include child spans from Strands Agents.

Alternatives Solutions

No response

Additional Context

No response