🐛 Bug Report: Whole message history is stringified into a single 'user prompt'

Which component is this bug for?

VertexAI Instrumentation

📜 Description

Passing a list of messages to generate_content combines them all into a single string and stores/presents them as a single user prompt instead of keeping them separate.

👟 Reproduction steps

import vertexai
from traceloop.sdk import Traceloop
from vertexai.generative_models import Content, GenerativeModel, Part

Traceloop.init()
vertexai.init()
model = GenerativeModel('gemini-1.5-flash-002')
response = model.generate_content(
    [
        Content(
            role='user',
            parts=[
                Part.from_text("What's 2+2?"),
            ],
        ),
        Content(
            role='assistant',
            parts=[
                Part.from_text('5'),
            ],
        ),
        Content(
            role='user',
            parts=[
                Part.from_text('really?'),
            ],
        ),
    ]
)

👍 Expected behavior

Attributes look something like:

{
  "gen_ai.prompt.0.role": "user",
  "gen_ai.prompt.0.content": "What's 2+2?\n",
  "gen_ai.prompt.1.role": "assistant",
  "gen_ai.prompt.1.content": "5\n",
  "gen_ai.prompt.2.role": "user",
  "gen_ai.prompt.2.content": "really?\n",
  "gen_ai.completion.3.role": "assistant",
  "gen_ai.completion.3.content": "Oops!  My apologies. 2 + 2 = 4.  I'm still under development and learning to perform these calculations correctly.\n"
}

👎 Actual Behavior with Screenshots

Attributes look like:

{
  "gen_ai.prompt.0.user": "role: \"user\"\nparts {\n  text: \"What\\'s 2+2?\"\n}\n\nrole: \"assistant\"\nparts {\n  text: \"5\"\n}\n\nrole: \"user\"\nparts {\n  text: \"really?\"\n}\n\n",
  "gen_ai.completion.0.role": "assistant",
  "gen_ai.completion.0.content": "Oops!  My apologies. 2 + 2 = 4.  I'm still under development and learning to perform these calculations correctly.\n"
}

🤖 Python Version

No response

📃 Provide any additional context for the Bug.

https://github.com/traceloop/openllmetry/blob/d92a50e9f5662d215e5808c8059ca9732480343e/packages/opentelemetry-instrumentation-vertexai/opentelemetry/instrumentation/vertexai/__init__.py#L124-L138

👀 Have you spent some time to check if this bug has been raised before?

• I checked and didn't find similar issue

Are you willing to submit PR?

None

Start in opentelemetry/instrumentation/vertexai/init.py at lines 124-138, then run the provided VertexAI generate_content reproduction. Trace how the list of Content messages becomes prompt attributes and preserve each message's role and content separately. Done means the attributes match the expected prompt.0, prompt.1, and prompt.2 structure rather than one stringified user prompt.