Sets the value of 'a' to 4, but also stores the timestamp from the source for when the notification was published

ExampleMongoDBSettingOfLastModifiedTimestap.ts

/** create connection pool to system DB in MongoDB
* assume we are using https://www.npmjs.com/package/mongodb
* assume we begin with:
{
  system_id: 123,
  a: 1,
  b: 2,
  c: 3
}

Receive new state with the following data:
{
  system_id: 123,
  a: 4,
  published: '2020-03-10T10:33:43.012Z'
}
*/
system_states_collection.update(
   { system_id: 123 },
   {
      $set: { 
        a: 4,
        'last_modified.a': Date('2020-03-10T10:33:43.012Z')
      },
   }
)

/** 
Resulting document: 
{
  system_id: 123,
  a: 4,
  b: 2,
  c: 3,
  last_modified: {
    a: '2020-03-10T10:33:43.012Z'
  }
}
*/