Set `operation` for RPC calls recorded via OpenTelemetry

### Problem Statement

We have special handling to set `operation` for OTEL spans in `SpanDescriptionExtractor` for HTTP and DB. We should add it for RPC as well.

Came up here https://github.com/getsentry/team-webplatform-meta/issues/32

### Solution Brainstorm

See https://github.com/getsentry/sentry-javascript/blob/f8047f66ac8abf64f992b5d681007383ea175276/packages/opentelemetry-node/src/utils/parse-otel-span-description.ts#L36-L44

首先定位 SpanDescriptionExtractor，並檢查其現有的 OpenTelemetry HTTP 和 DB span 處理邏輯。比較所參照的 sentry-javascript 解析邏輯，然後確認 RPC span 也會取得 operation 值；如果在附近找到相關測試，則新增或更新這些測試。

由索引模型根據 Issue 內容生成。