feat: Add custom telemetry context upon client creation (#31)
* feat: Add custom telemetry context upon client creation
- **Description:** Add custom user agent to Vertex AI SDK initialization. Allows API usage metrics collection.
- Follow-up to https://github.com/langchain-ai/langchain/pull/12168
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Co-authored-by: Leonid Kuligin <lkuligin@yandex.ru>
