I think (!?) that this API is not yet exposed through APIs Explorer:

E.g. The following 404s (NOT_FOUND)

API=apkeys
VER=v2alpha1

curl https://www.googleapis.com/discovery/v1/apis/${API}/${VER}/rest

Unfortunately, until it is, (there's no discovery document and) the API Client library is unable to auto-generate the SDK for it.

It's unclear to me whether this is policy or an oversight.

I recommend you pester the Cloud SDK team on Google's Issue Tracker (for Cloud SDK)

Note:

If you append --log-http to (any) gcloud command, it will display the underlying REST calls for the command. Absent a Google-provided SDK for these methods, you could introspect the API and code the REST calls directly:

gcloud alpha services api-keys create ... \
--project=${PROJECT} \
--log-http

Yields:

==== request start ====
uri: https://apikeys.googleapis.com/v2alpha1/projects/${PROJECT}/keys?alt=json
method: POST
== headers start ==
b'accept': b'application/json'
b'authorization': b'Bearer ya29...'
== headers end ==
== body start ==
== body end ==

New: It is now possible to create API Key using @google-cloud/apikeys package

For anybody wondering this recently.

This is the example provided from official Github:

https://github.com/googleapis/google-cloud-node/blob/main/packages/google-api-apikeys/samples/generated/v2/api_keys.create_key.js

// Imports the Apikeys library
const {ApiKeysClient} = require('@google-cloud/apikeys').v2;

// Instantiates a client
const apikeysClient = new ApiKeysClient();

async function callCreateKey() {
  // Construct request
  const request = {
    parent,
    key,
  };

  // Run request
  const [operation] = await apikeysClient.createKey(request);
  const [response] = await operation.promise();
  console.log(response);
}

callCreateKey();

Check out the related document as well https://cloud.google.com/nodejs/docs/reference/apikeys/latest