Graphite: Expand metric names for variables

[ifrost]

What this PR does / why we need it:

Variable query for Graphite allows to search for metrics, however it's quite limited because results contain possible values but occurring only at the last level of the query (please check the example below). This PR introduces "expand" function allowing to use expand API to get the full name of matching metrics. In combination with regex it will allow to extract any part of the metric name.

Example:

Metrics:

• prod.servers.server001.cpu
• prod.servers.server002.cpu
• test.servers.server001.cpu

Search results: (normal query is already available, expanded query support is added in this PR)

normal query	results	expanded query	expanded results
*	prod, test	expand(*)	prod, test
*.servers	servers	expand(*.servers)	prod.servers, test.servers
test.servers	servers	expand(test.servers)	test.servers
*.servers.*	server001,server002	expand(*.servers.*)	prod.servers.server001, prod.servers.server002, test.servers.server001
test.servers.*	server001	expand(test.servers.*)	test.servers.server001
*.servers.*.cpu	cpu	expand(*.servers.*.cpu)	prod.servers.server001.cpu, prod.servers.server002.cpu, test.servers.server001.cpu

⚠️ Note: "normal query" is the same as "expand query" with a regex matching the last part of the name.

Why is it important?

The available implementation allows to handle only simpler (though most common) cases where only the last part of the metric should become a variable value. However, it's not possible extract other parts of metric names.

Imagine you needed a dropdown to select the environment (prod or test in the example above). Normal search could capture the top level name but only of all metrics you have. Let's say besides prod.* and test.* metrics you had statsd.*. With normal query "statsd" would also be captured. With "expand" it's possible to write do this:

Query: expand(*.servers)
Regex: /^(\w+).*/

Expand will first find only metrics that have servers in the second level and then regex will capture only environment names.

How to test it?

Run: make devenv sources=graphite
Send some sample data: https://gist.github.com/ifrost/ada07138775d5295fba169d27abddf8d

Create a dashboard with following query: $env.servers.$server.cpu

Create variables:

env:

and server:

Which issue(s) this PR fixes:

Fixes #2343

Special notes for your reviewer: Same example is included in updated docs.

[achatterjee-grafana]

Is this for V8.0?

[ifrost]

Is this for V8.0?

Yes! Sorry, forgot about the milestone. Updated and adding "add to changelog" label.

[achatterjee-grafana]

Added change suggestions.

[achatterjee-grafana]

Doc looks ok, we can refine it further if needed.

[gabor]

looks good to me 👍 !