chore: added README

README.md

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+# V metrics
+
+This library defines an interface for implementing a metrics collector & an
+accompanying exporter interface for serializing the data in some way. A
+thread-safe implementation of the collector interface is provided, which
+utilizes a combination of atomic operations & `lock`/`rlock` to ensure the data
+structure can be safely used concurrently. An implementation of a
+Prometheus-compatible exporter is planned.
+
+This library takes a lot of inspiration from the Rust
+[metrics crate](https://docs.rs/metrics/latest/metrics/index.html).
+
+Note that this library is still in development, and stuff might still change.
+
+## Metric types
+
+Just like the Rust crate, this library defines three types of metrics.
+
+### Counter
+
+A counter is a `u64` value that can only be monotonically incremented. One
+example of such value is the amount of requests a web server has processed.
+
+### Gauge
+
+A gauge is an `f64` value describing a metric that can arbitrarily increase or
+decrease over time. It can be freely added to, subtracted from or plainly set
+to an absolute value. This could for example be used to track memory usage.
+
+### Histogram
+
+A histogram is an arbitrarily growing list of `f64` values. These values can
+then be used to provide statistical analysis over time. The processing of these
+values is left to the exporter to decide. The metrics collector solely stores
+these values.
+
+## Interfaces
+
+As mentioned above, this library defines two interfaces. These can either be
+used by using one of the provided implementations, or a user can freely
+implement their own versions.
+
+### `MetricsCollector`
+
+A metrics collector is responsible for receiving and storing the types of
+metrics. Two implementations are provided, namely `DefaultCollector` and
+`NullCollector`. The former is the above-mentioned thread-safe implementation,
+while the latter is an implementation that does absolutely nothing. The usecase
+I have in mind for this seemingly useless version is when it's configurable
+whether a piece of software should collect metrics or not. If the answer is no,
+the developer could simply use a `NullCollector`, without having to change any
+of the other code. All calls to the metrics collector will just get silently
+ignored.
+
+### `MetricsExporter`
+
+A metrics exporter is responsible for converting the data stored in a metrics
+collector into some format that can then be processed by some other means. The
+classic example would be exporting to a Prometheus-compatible text format,
+which could then be ingressed by Prometheus and processed further.