doc: fill README.md

Closes #5

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - The role implementation (#6).
+- A documentation for the role to README.md (#5).
 
 ### Fixed
 

README.md

@@ -1,2 +1,145 @@
 # metrics-export-role
-The Tarantool 3 role for metrics export via HTTP
+
+`roles.metrics-export` is a role for Tarantool 3. It allows to export metrics
+from Tarantool 3. For now only export via HTTP is supported.
+
+## Usage
+
+1. Add the `metrics-export-role` package to dependencies in the .rockspec file.
+
+```Lua
+dependencies = {
+    ...
+    'metrics-export-role == 0.1.0-1',
+    ...
+}
+```
+
+And re-build an application:
+
+```shell
+tt build
+```
+
+Be careful, it is better to use a latest release version.
+
+2. Enable and [configure](https://www.tarantool.io/en/doc/latest/concepts/configuration/)
+the `roles.metrics-export` role for a Tarantool 3 instance.
+
+```yaml
+groups:
+  group-001:
+    replicasets:
+      replicaset-001:
+        instances:
+          master:
+            roles: [roles.metrics-export]
+            roles_cfg:
+              roles.metrics-export:
+                http:
+                - listen: 8081
+                  endpoints:
+                  - path: /metrics/json
+                    format: json
+                  - path: /metrics/prometheus/
+                    format: prometheus
+                - listen: 'my_host:8082'
+                  endpoints:
+                  - path: /metrics/prometheus
+                    format: prometheus
+                  - path: /metrics/json/
+                    format: json
+```
+
+In the example above, we configure two HTTP servers on `0.0.0.0:8081` and
+`my_host:8082`. The servers will be running on the `master` Tarantool
+instance.
+
+Each server has two endpoints:
+
+* `/metrics/json` exports metrics with JSON format.
+* `/metrics/prometheus` exports metrics with Prometheus-compatible format.
+
+## Configuration
+
+The role configuration at the top level could be described as:
+
+```yaml
+export_target: opts
+```
+
+### http target
+
+For now only `http` target is supported. The target allows to export metrics via
+HTTP-servers. The target could be configured as an array of servers.
+
+Each server must have `listen` param that could be a port (number) or a string
+in the format `host:port`. The address will be used by HTTP server to listen
+requests.
+
+Each server could have `endpoints` param as an array of endpoints to process
+incoming requests. An individual endpoint can be described as:
+
+```yaml
+- path: /path/to/export/on/the/server
+  format: format_to_export
+```
+
+For now only `json` and `prometheus` formats are supported.
+
+Let's put it all together now:
+
+```yaml
+roles_cfg:
+  roles.metrics-export:
+    http:
+    - listen: 8081
+      endpoints:
+      - path: /metrics
+        format: json
+      - path: /metrics/prometheus/
+        format: prometheus
+    - listen: '127.0.0.1:8082'
+      endpoints:
+      - path: /metrics/
+        format: json
+```
+
+With this configuration, metrics can be obtained on this machine with the
+Tarantool instance as follows:
+
+```shell
+curl -XGET 127.0.0.1:8081/metrics
+# Metrics will be returned in JSON format.
+curl -XGET 127.0.0.1:8081/metrics/prometheus
+# Metrics will be returned in Prometheus-compatible format.
+curl -XGET 127.0.0.1:8082/metrics
+# Metrics will be returned in JSON format.
+```
+
+## Development
+
+First you need to clone the repository:
+
+```shell
+git clone https://github.com/tarantool/metrics-export-role
+cd metrics-export-role
+```
+
+After that you need to install dependencies (`tt` is required):
+
+```shell
+make deps
+```
+
+At this point you could run tests (`tarantool` 3 is required):
+
+```shell
+make test
+```
+
+And a linter:
+
+```shell
+make luacheck
+```