User Story

As a driver engineer,
I want the MongoClient's cached client metadata to be internal,
So that I modify it without worrying about semver.

As a driver engineer,
I want to remove the legacy `metadata` property on the MongoClient that is no longer used in favor of the `extendedMetadata` and rename `extendedMetadata` as `metadata`,
So that I only have a single source of client metadata per mongoclient.

Background: Prior to NODE-5454, client metadata was built synchronously during options parsing and stored on the MongoClient. NODE-5454 required file access, which necessitated async code. The solution we chose was to instead cache a promise that resolves to client metadata on the MongoClient and await it where we use it (this occurs in `connect()`, which is asynchronous).

However, for whatever reason, `MongoClient.metadata` is public so we could not modify this property to change it to a promise. So, we added a field called `extendedMetadata` on the MongoClient. Now the MongoClient stores two fields:

  metadata: ClientMetadata;
  /** @internal */
  extendedMetadata: Promise<Document>;

We currently construct these options like so:

  mongoOptions.metadata = makeClientMetadata(mongoOptions);

  mongoOptions.extendedMetadata = addContainerMetadata(mongoOptions.metadata).then(
    undefined,
    squashError
  ); // rejections will be handled later

As a part of this ticket we would like to:

1. remove the unused `metadata` property from the MongoClient and instead store the metadata promise as `metadata`.
2. make sure the interface `ClientMetadata` is accurate and includes container runtime fields (I don't think it does now)
3. make the ClientMetadata interface private
 

User Impact

• should be none, unless users are using the `metadata` property (I can't see why they would?)

Acceptance Criteria

Implementation Requirements

• Mark ClientMetadata interface as internal
• Audit the ClientMetadata metadata interface to make sure it is accurate and matches the spec. Fix any errors encountered.
• Remove the `metadata` property from the MongoClient and rename `extendedMetadata` as `metadata` on the client. (this will require a few code changes, TS should catch this. anywhere we use `extendedMetadata` needs to be updated).
• Move container metadata logic into makeClientMetadata

Testing Requirements

• Make sure existing tests pass

Documentation Requirements

• n/a