feat: add ability to parse promql with grafana templates inside

[com30n]

Hello!

I'm writing a script, which can download grafana dashboard, patch promql expressions (add some labels, rename labels, etc),
and push it back to grafana.

For parsing promql I'm using the lexer, and while doing some tests I faced the problem:
at some dashboards promql expressions contain grafana templates.

here is an example of the expression:
vault_route_create_${mountpoint}_{quantile="0.99"}

and it made a problem while parsing it, because the Metrics regex doesn't contain an expression to parse strings inside ${} symbols.

So, the little fix adds the ability to parse promql expressions that contain grafana templates inside.

P.S. While making the changes I found that the tests is not using local PromQLLexer class, so I added a new test, which is testing the new changes, with new test cases.

[pabluk]

Hi @com30n, sorry for the late reply and thanks for your contribution!

It's an interesting use case, and it could be great to have support for that kind of expressions from Grafana.

[pabluk]

It could be better to implement this specific rules creating a subclass of PromQLLexer, something like:

class PromQLGrafanaLexer(PromQLLexer):
    name = "PromQLGrafana"
    aliases = ["promqlgrafana"]
    filenames = ["*.promql"]

    # only overwriting the needed tokens
    tokens["root"] = {...}  # new rules for Grafana
...

and adding the class to:

__all__ = ["PromQLLexer", "PromQLGrafanaLexer"]

that will allow to choose a specific lexer to users only wanting pure promql or Grafana one.

[pabluk]

By the way, requirements were updated on #16 and #17 so you can rebase this branch to pass tests.