Pushes an object containing 'saved_at' with today's date, 'saved_by' with a user name into the save_history property

ExampleMongoDB$pushStatement.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
}
*/

system_states_collection.update(
  { system_id: 123 },
     {
        $set: { a: 4 },
        $push: {
          save_history: {
              saved_at: new Date(),
              saved_by: 'SomeUserName'
          }
        }
     }
)

/** 
Resulting document: 
{
  system_id: 123,
  a: 4,
  b: 2,
  c: 3,
 {
    save_history: [{
        saved_at: '2020-03-08T10:27:21.230Z',
        saved_by: 'SomeUserName'
      }
    ]
  }
}
*/

system_states_collection.update(
  { system_id: 123 },
  {
      $set: { b: 5 },
      $push: {
        save_history: {
            saved_at: new Date(),
            saved_by: 'SomeOtherUserName'
        }
      }
   }
)

/** 
Resulting document: 
{
  system_id: 123,
  a: 4,
  b: 5,
  c: 3,
 {
    save_history: [
      {
        saved_at: '2020-03-08T10:27:21.230Z',
        saved_by: 'SomeUserName'
      },
      {
        saved_at: '2020-03-09T10:27:21.230Z',
        saved_by: 'SomeOtherUserName'
      }
    ]
  }
}
*/