Why can't {} be used to match every metric?

[EdSchouten]

I've noticed that Prometheus contains some smartness that prevents you from running queries that match every metric. For example, running the query {} prints the following error:

Error executing query: parse error at char 2: vector selector must contain label matchers or metric name

Let's add some smartness to work around this by running {__name__=~".*"}:

Error executing query: parse error at char 16: vector selector must contain at least one non-empty matcher

You could ask yourself: why would you want to run a query that matches every metric? Well, I just want to get a rough idea of the number of metrics our Prometheus setup stores by job. Ideally I'd just want to run count({}) by (job), but now I'm forced to use workarounds like count({__name__=~"..*"}) by (job) (notice the ..*).

[brian-brazil]

This is on purpose to reduce the chances of users accidentally hosing their Prometheus, as it's very easy to end up with such an expression while playing around.

[mprasil]

@EdSchouten use ".+" if you find "..*" too hack-ish.

[lock]

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