Update metric definitions and improve metric retrieval

[bripkens]

This PR…

• Updates the metric definitions.
• Ensures that we transmit both from and to for the /api/snapshots/context API call. This is necessary so that we can properly execute the search queries.
• Adds a checkbox "New application model enabled" and transmits the value true/false to the server when executing searches.
• Ensures that form labels are clickable within the query builder.
• Realigns the form so that the input fields are vertically stacked. This makes the form more accessible on lower resolution screens.
• Extends the contribution guides with a hint to avoid some unnecessary googling.
• Ensures thats the functional test for the automatic creation of the datasource work in higher latency environments.
• Adds additional logging to highlight the fact the API server URL and API token can be configured via environment variables for dev mode start.

I recommend looking at this with ignored whitespace changes :)

https://github.com/instana/instana-grafana-datasource/pull/5/files?utf8=%E2%9C%93&diff=split&w=1

Screenshots

Query string parameters used for the search APIs:

New items within the form:

[JeroenSoeters]

Had a discussion with Raphael and we thought it might be better to move the "New Application Model" button to the datasource settings instead of requiring the user to provide it with every query as this will be either enabled for the tenant unit or not. We don't expect users to be switching back and forth.

[bripkens]

Moved it to datasource configuration. This should address all the open remarks.