feat: add current span enrichment helpers

[shreyas-n-harness]

Currently there is no way in which users can ingest custom data (with custom labels), this PR adds a way to ingest per span attribute

Usecases

• CACM users adding tenant data per trace
• Agent action resolved per trace. Example: created, updated, deleted, no_action

Changes

• Add set_span_attribute() and set_span_attributes() public APIs.
• Enrich only the active recording span.
• Preserve native OpenTelemetry attribute types.
• No baggage, child-span inheritance, or downstream propagation.
• Document usage, timing, precedence, and sampling constraints.
• Add focused unit tests.

Usage

from google import genai
from opentelemetry import trace

from harness_sdk import set_span_attributes

client = genai.Client(
    vertexai=True,
    project="my-gcp-project",
    location="us-central1",
)

tracer = trace.get_tracer(__name__)


def generate_answer(tenant_id: str, prompt: str) -> str:
    with tracer.start_as_current_span("tenant.vertex.request"):
        set_span_attributes({
            "tenant.id": tenant_id,
            "agent.action.type": "generate-answer",
        })

        response = client.models.generate_content(
            model="gemini-2.5-flash",
            contents=prompt,
        )

        return response.text

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