beater/middleware: update fields to be ECS-compliant

[axw]

We should update the logging middleware to use ECS-compliant field names and values:

apm-server/beater/middleware/log_middleware.go

Line 34 in c0f4767

func LogMiddleware() Middleware {

"method" -> "http.method", "URL" -> "http.url", etc.

[simitt]

Depends on elastic/beats#18820