Allow users to retrieve computed metrics

[panyuenlau]

Problem statement

Currently, user is unable to retrieve metrics that was computed against a particular session.

Proposed solution

1. Write computed metrics back to the DB.
2. Expose an API endpoint for users to query computed metrics based on session id.
3. Provide an example Grafana dashboard template to visualize the computed metrics so users can use to build or extend their Grafana dashboard. For example:

[Castorche]

Hey @panyuenlau,
regarding your point 1, this PR #96 will give you more control on which metrics you store in the DB (there was an ON/OFF behaviour with the METRICS_CACHE_ENABLED env variable, if set to True, every metrics was stored and none otherwise). Now, you can add an option in your query to your request to the MCE as the following:

"metric_options": { "write_to_db": true }

And the results of your request will be stored into the DB.
The README was updated to make this cleared as well.

I will propose an example of grafana dashboard template to display stored metrics as well.

[Castorche]

Hey @panyuenlau, we merged #97, so you have now a Grafana template to display a computed metric for a given session.

[panyuenlau]

Hey @panyuenlau, we merged #97, so you have now a Grafana template to display a computed metric for a given session.

Thank you for the speedy response for the feature request! Will definitely try it out!

Can you please drop a tag to include the changes in #96 so we can experience it on our end?