Include Process ID

This a the default format of log generated by spring boot app using logback

`2020-10-28 11:03:27.265 DEBUG 11672 --- [nio-8082-exec-1] c.b.estimator.domain.MarketEstimate : Value Estimation of Model : Toyota`

The ECS format generated in json is as follow

```
{
"@timestamp": "2020-10-28T11:03:27.265Z",
"log.level": "DEBUG",
"message": "Value Estimation of Model : Toyota",
"service.name": "car-value-estimator",
"event.dataset": "car-value-estimator.log",
"process.thread.name": "http-nio-8082-exec-2",
"log.logger": "com.bvader.estimator.domain.MarketEstimate",
"event.module": "car-value-estimator",
"event.category": "log",
"log.origin": {
"file.name": "MarketEstimate.java",
"function": "calculateEstimate",
"file.line": 29
}
}
```

As you can see the filed Process ID with value 11672 is missing

Start by tracing how the ECS JSON output maps fields from the Spring Boot Logback record, then compare it with the default log line where process ID 11672 appears. Done means the generated ECS JSON includes the process ID as a process field; no source file or test is named in the issue.

Written by the indexing model from the issue text.