Remap support for the filter transform

The filter transform for Vector enables you to winnow down a stream of events to only those that match a specified condition.

Previously, the filter transform required you to specify conditions using check_fields. But now you can use Vector Remap Language expressions in the remap transform to specify those conditions. This should make using filter more natural and expressive.

Example

The example configuration below shows the same filter transform using the old system (check_fields) and the new system (remap):

[transforms.filter_out_non_critical]
type = "filter"
inputs = ["http-server-logs"]f

# Using check_fields
condition.type = "check_fields"
condition.message.status_code.ne = 200
condition.message.severity.ne = "info"
condition.message.severity.ne = "debug"

# Using remap
condition.type = "remap"
condition.source = '.status_code != 200 && !includes(["info", "debug"], .severity)'