add rust counter, gauge and histogram links (#2538)

Signed-off-by: Ali Yousefi Sabzevar <[email protected]>

Author: iamaliyousefi

content/docs/concepts/metric_types.md

@@ -29,6 +29,7 @@ Client library usage documentation for counters:
    * [Python](https://prometheus.github.io/client_python/instrumenting/counter/)
    * [Ruby](https://github.com/prometheus/client_ruby#counter)
    * [.Net](https://github.com/prometheus-net/prometheus-net#counters)
+   * [Rust](https://docs.rs/prometheus-client/latest/prometheus_client/metrics/counter/index.html)
 
 ## Gauge
 
@@ -46,6 +47,7 @@ Client library usage documentation for gauges:
    * [Python](https://prometheus.github.io/client_python/instrumenting/gauge/)
    * [Ruby](https://github.com/prometheus/client_ruby#gauge)
    * [.Net](https://github.com/prometheus-net/prometheus-net#gauges)
+   * [Rust](https://docs.rs/prometheus-client/latest/prometheus_client/metrics/gauge/index.html)
 
 ## Histogram
 
@@ -84,6 +86,7 @@ Client library usage documentation for histograms:
    * [Python](https://prometheus.github.io/client_python/instrumenting/histogram/)
    * [Ruby](https://github.com/prometheus/client_ruby#histogram)
    * [.Net](https://github.com/prometheus-net/prometheus-net#histogram)
+   * [Rust](https://docs.rs/prometheus-client/latest/prometheus_client/metrics/histogram/index.html)
 
 ## Summary