add prometheus submodule readme

Author: Eduard Llull

prometheus/README.md

@@ -0,0 +1,40 @@
+fastbreaker
+===========
+
+[![Go Reference](https://pkg.go.dev/badge/github.com/bluekiri/fastbreaker/prometheus.svg)](https://pkg.go.dev/github.com/bluekiri/fastbreaker/prometheus) [![CI](https://github.com/bluekiri/fastbreaker/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/bluekiri/fastbreaker/actions/workflows/ci.yml)
+
+[fastbreaker.prometheus](https://github.com/bluekiri/fastbreaker/prometheus) simplifies regitering metrics generated from [fastbreaker](https://github.com/bluekiri/fastbreaker) into [prometheus](https://prometheus.io/).
+
+Installation
+------------
+
+```
+go get github.com/bluekiri/fastbreaker/prometheus
+```
+
+Usage
+-----
+
+The function `prometheus.RegisterMetricsToDefaultRegisterer` registers the `fastbreaker.FastBreaker` metrics with the default Registerer.
+
+The function `prometheus.RegisterMetrics` registers the `fastbreaker.FastBreaker` metrics with the provided Registerer.
+
+The function `RegisterMetricsWithFactory` registers the `fastbreaker.FastBreaker` metrics with the provided `promauto.Factory`.
+
+All three functions return an error if the circuit breaker name is not a valid UTF-8 string.
+
+Example
+-------
+
+```go
+var cb fastbreaker.FastBreaker
+
+prometheus.RegisterMetricsToDefaultRegisterer("my-circuit-breaker", cb)
+```
+
+License
+-------
+
+The MIT License (MIT)
+
+See [LICENSE](https://github.com/bluekiri/fastbreaker/blob/master/LICENSE) for details.