Support for metadata labels in VertexAI library

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**Is your feature request related to a problem? Please describe.**
Java library for VertexAI does not offer support for adding labels when making `generateContent` calls. This is really useful feature for tracking costs for different labels. And also this is supported by REST call and by python library. Link - https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/add-labels-to-api-calls

**Describe the solution you'd like**
Solution could be the same as in python library
```
GenerativeModel::generateContent(Content content, Map labels)
```

This is the example from python library:
```
prompt = "What is Generative AI?"
response = model.generate_content(
prompt,
# Example Labels
labels={
"team": "research",
"component": "frontend",
"environment": "production",
},
)
```

**Describe alternatives you've considered**
Unfortunately no alternatives in this library. If I want to use labels I need to switch to REST calls and discard this library.