Remap support for the `reduce` transform (multi-line logs)

The reduce transform for Vector enables you to reduce multiple log events into a single event, which is useful for logs that are split across multiple lines, such as JVM stack traces. With reduce you can specify both a merge strategy for the events and a condition for specifying when a group of events either starts or ends (that group is then collapsed, i.e. reduced into a single event).

Previously, the reduce 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 reduce more natural and expressive.

Example

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

 [transforms.merge_stack_trace]
 type = "reduce"
 inputs = ["jvm-logs"]
 merge_strategies.message = "concat_newline"

 # Using check_fields
-starts_when.type = "check_fields"
-starts_when.message.regex = "/^\\w.*/"
-starts_when.severity.eq = "info"

 # Using remap
+starts_when.type = "remap"
+starts_when.source = 'match(string!(.message), r'^\\w.*') && .severity == "info"'