Show usage stats prompt when it's enabled.
Current UX are:
* The usage stats enabled or disabled message is shown every time in both terminal and dashboard.
* If users don't explicitly enable or disable usage stats, the first time they start a ray cluster interactively, they will be asked to confirm and will enable if no user action within 10s. If it's non-interactive, collection is enabled by default without confirmation.
* ray.init() doesn't collect usage stats
* Usage stats can be disabled via three approaches: 1. RAY_USAGE_STATS_ENABLED env var, 2. ray xxx --disable-usage-stats, 3. ray disable-usage-stats
Fix bug from the previous fixes.
Add more tests
Stop using m5.xlarge (not supported now)
There are 2 hard blockers from the infra: 1. Large size disk is not supported. 2. m5.xlarge is not supported. Both are considered as a high priority to be fixed soon.
This is the second PR to implement usage stats on Ray. Please refer to the file usage_lib.py for more details.
The full specification is here https://docs.google.com/document/d/1ZT-l9YbGHh-iWRUC91jS-ssQ5Qe2UQ43Lsoc1edCalc/edit#heading=h.17dss3b9evbj.
This adds a dashboard module to enable usage stats. **Usage stats report is turned off by default** after this PR. We can control the report (enablement, report period, and URL. Note that URL is strictly for testing) using the env variable.
## NOTE
This requires us to add `requests` to the default library. `requests` must be okay to be included because
1. it is extremely lightweight. It is implemented only with built-in libs.
2. It is really stable. The project basically claims they are "deprecated", meaning no new features will be added there.
cc @edoakes @richardliaw for the approval
For the HTTP request, I was alternatively considered httpx, but it was not as lightweight as `requests`. So I decided to implement async requests using the thread pool.
