Describe

Let's break down what happened in the last exercise. You created a counter: a monotonically increasing value.
Once it has been created, its value can only be incremented.

You can create a counter using the counter! macro. You can then call increment on the Counter returned by the macro to increment its value.

Counter exposes another method, absolute, which is a bit special: it's primarily designed to initialize a counter sequence, specifying a non-zero initial value. Beware: recorders will enforce the monotonicity property for counters, so you can't rely on absolute to artificially decrement an existing counter.

When using counter!, the Recorder implementation is expected to:

• Create a new counter with the name you specified, if one doesn't exist;
• Retrieve the counter, if one exists.

UPSERT behaviour, for the SQL-minded among us.

Metadata

It can be useful to add metadata to your counters (and metrics in general).
You can rely on the describe_* macros offered by the metrics! crate: they let you add a unit (e.g. capture that the counter value represents bytes or seconds) and a description (which recorders can then choose to expose when metrics are exported/analyzes).

Exercise

The exercise for this section is located in 03_metrics/02_describe