Initial Gravel Gateway Support

[brettimus]

This is a change to support autometrics-ts when used with the gravel aggregation gateway.

Allow a clearmode="family" label on the build_info metrics

Aggregating a gauge like build_info requires a special label.

We want to clear previous build_info metrics when new ones are pushed.

Grave Gateway requires that you add a clearmode label to do this.

Objective labels must always be present

Gravel Gateway will reject two metrics of the same name if one of them has a subset of the labels of the other.

This means to be able to support functions that have objectives, as well as functions that don't, we need to default to setting the objective_* labels to the empty string.

Looking to the Future

• Add support for specifying the job name
• Check that pushing build_info with a new job name does not clear all existing build_info metrics with different job names