Add span_id as a first-class filter in the Logs explorer

### Problem Statement

Currently, the Logs explorer supports filtering by `trace_id`, which returns all logs across an entire trace.

However, there is no straightforward way to filter logs scoped to a specific span within that trace.
When diagnosing slow requests, it is often necessary to isolate logs associated with a single span so please consider adding `span_id` as a first-class, suggested filter attribute in the Logs explorer, alongside `trace_id`.

### Solution Brainstorm

_No response_

### Product Area

Explore

Start by tracing how the Logs explorer currently exposes and applies the trace_id filter, including any tests for suggested filter attributes. Determine where span_id should be added alongside trace_id and verify that filtering scopes results to the selected span. Done means span_id is a first-class suggested filter in Explore and its behavior is covered by tests.

Written by the indexing model from the issue text.