Incomplete meta in the event

[sagar1312]

What happened:
Whenever a build is updated, its meta is merged with event meta. But, this merge is shallow (one level). Due to this nested meta set by previous builds of this event could be lost when the latest build meta is merged. This is more common when the builds are triggered and executed in parallel.

For Example:
Build meta

{
  "build": {
    ...
  },
  "commit": {
    ...
  },
  "event": {
    ...
  },
  "example": {
    "prod-blue-test1": 91,
    "prod-blue-test2": 92,
    "prod-blue-test3": 93,
    "prod-blue-test5": 95
  }
}

Event meta before merge

{
  "build": {
    ...
  },
  "commit": {
    ...
  },
  "event": {
    ...
  },
  "example": {
    "prod-blue-test1": 91,
    "prod-blue-test2": 92,
    "prod-blue-test3": 93,
    "prod-blue-test4": 94
  }
}

Event meta after merge

{
  "build": {
    ...
  },
  "commit": {
    ...
  },
  "event": {
    ...
  },
  "example": {
    "prod-blue-test1": 91,
    "prod-blue-test2": 92,
    "prod-blue-test3": 93,
    "prod-blue-test5": 95
  }
}

example.prod-blue-test4 is lost during the merge.

What you expected to happen:
build meta should be deep merged (recursively) into event meta.

Event meta after merge

{
  "build": {
    ...
  },
  "commit": {
    ...
  },
  "event": {
    ...
  },
  "example": {
    "prod-blue-test1": 91,
    "prod-blue-test2": 92,
    "prod-blue-test3": 93,
    "prod-blue-test4": 94,
    "prod-blue-test5": 95
  }
}

How to reproduce it:
Refer to this pipeline https://cd.screwdriver.cd/pipelines/15117/events/819320
After the event is completed, the event meta does not contain all the data set by all the individual builds.