Open a Metasys server-sent-events stream and subscribe to live COV attribute updates

Documented steps

1. Authenticate and obtain a Bearer token.
2. GET {base}/stream with Accept: text/event-stream. Authenticate either with the Authorization: Bearer header or, from a browser EventSource that cannot set headers, with ?access_token={accessToken} on the URL.
3. Read the first SSE frame. On a new connection it is 'event: hello' and its 'data' field is the stream id (a GUID). Keep that stream id - every subscription is bound to it.
4. Subscribe an attribute: GET {base}/objects/{objectId}/attributes/{attributeId} with header METASYS-SUBSCRIBE: {streamId}. The response is 202 Accepted, not the value.
5. Capture the METASYS-SUBSCRIPTION-LOCATION response header - it is the URL of the created subscription and the only handle you have for cancelling it.
6. To subscribe many points at once, POST /objects/batch with the METASYS-SUBSCRIBE header set; every sub-request in the batch becomes a subscription on that stream.
7. Consume events off the stream. Expect periodic 'heartbeat' events whose data is a JSON string of the current UTC time, e.g. "2021-02-16T16:21:10.23Z"; treat a long gap in heartbeats as a dead connection.
8. Refresh your access token before it expires while the stream is open. Metasys updates the stream expiration to match the new token's expiry, so a refreshed token keeps the stream alive.
9. Unsubscribe by issuing DELETE to the URL you saved from METASYS-SUBSCRIPTION-LOCATION.
10. Keep total streamed points at or below 1,500 per engine, and raise the COV Min Time attribute on chatty points (at both the point and the engine) to cut event volume.
11. Reference: https://jci-metasys.github.io/api-landing/api/v6-16 (Metasys REST API, 'Version 6 for 16.0', OpenAPI 3.0.3). Doc set index: https://jci-metasys.github.io/api-landing/ | changelog: https://jci-metasys.github.io/api-landing/guides/api-changelog/ | version support: https://jci-metasys.github.io/api-landing/guides/version-support-matrix/ | polling and streaming limits: https://jci-metasys.github.io/api-landing/guides/attribute-access-guidelines/