Fix filter for Map type LogLabels

[waterdrag0n]

Fix filter not working when adding via + button in Explore logs

#1369 introduced OTel attribute support but broke existing Map/JSON type filter logic.

Problem 1: Empty mapKey

When clicking + button on a log field from a Map type column (e.g., LogAttributes as labels) in Explore, the filter was created with an empty key.

• Expected: labels['key'] = 'value'
• Actual: labels = 'value'

Fix: Fallback to columnName when mapKey is not set by OTel prefix parsing.

Problem 2: Empty columnType

When OTel mode is enabled, columns were set without type information, causing Map columns to be incorrectly treated as JSON.

• Expected: labels['key'] = 'value' (Map syntax)
• Actual: labels.`key` = 'value' (JSON syntax)

Fix: Fetch column type from table schema in useOtelColumns.

Related: #1369 (comment)