update documentation for new operating mode

CHANGELOG.md

@@ -6,6 +6,9 @@ New features:
   multiple times and take the average, to avoid alerts because of a one-time
   mismeasurement.
 - The NTP server name is now reported as a metric label.
+- The option `-ntp.source` has been added. With `-ntp.source http`, the NTP
+  server is not defined through command-line options, but through query
+  parameters on the HTTP GET request.
 
 Changes:
 

README.md

@@ -5,42 +5,54 @@ of that node's clock against a given NTP server.
 
 ## Installation
 
-To build the binary:
+Compile `make && make install` or `docker build`. The binary can also be
+installed with `go get`:
 
 ```bash
-make
+go get github.com/sapcc/ntp_exporter
 ```
 
-The binary can also be installed with `go get`:
+## Usage
 
-```bash
-go get github.com/sapcc/ntp_exporter
+Common command-line options:
+
+```
+-ntp.source string
+   source of information about ntp server (cli / http). (default "cli")
+-version
+   Print version information.
+-web.listen-address string
+   Address on which to expose metrics and web interface. (default ":9559")
+-web.telemetry-path string
+   Path under which to expose metrics. (default "/metrics")
 ```
 
-To build the Docker container:
+### Mode 1: Fixed NTP server
 
-```bash
-make GOLDFLAGS="-w -linkmode external -extldflags -static" && docker build .
+By default, or when the option `-ntp.source cli` is specified, the NTP server
+and connection options is defined by command-line options:
+
+```
+-ntp.measurement-duration duration
+   Duration of measurements in case of high (>10ms) drift. (default 30s)
+-ntp.protocol-version int
+   NTP protocol version to use. (default 4)
+-ntp.server string
+   NTP server to use (required).
 ```
 
-## Usage
+### Mode 2: Variable NTP server
+
+When the option `-ntp.source http` is specified, the NTP server and connection
+options are obtained from the query parameters on each `GET /metrics` HTTP
+request:
+
+- `server`: NTP server to use
+- `protocol`: NTP protocol version (2, 3 or 4)
+- `duration`: duration of measurements in case of high drift
+
+For example:
 
-Command-line options: (Only `-ntp.server` or `-ntp.source` is required.)
-
-```plain
-Usage of ntp_exporter:
-  -ntp.measurement-duration duration
-     Duration of measurements in case of high (>10ms) drift. (default 30s)
-  -ntp.protocol-version int
-     NTP protocol version to use. (default 4)
-  -ntp.server string
-     NTP server to use (required).
-  -ntp.source string
-     source of information about ntp server (cli / http). (default "cli")
-  -version
-     Print version information.
-  -web.listen-address string
-     Address on which to expose metrics and web interface. (default ":9559")
-  -web.telemetry-path string
-     Path under which to expose metrics. (default "/metrics")
+```sh
+$ curl 'http://localhost:9559/metrics?server=ntp.example.com&protocol=4&duration=10s'
 ```