Improve prometheus plugin

closes #707

plugins/inputs/prometheus/README.md

@@ -0,0 +1,75 @@
+# Prometheus Input Plugin
+
+The prometheus input plugin gathers metrics from any webpage
+exposing metrics with Prometheus format
+
+### Configuration:
+
+Example for Kubernetes apiserver
+```toml
+# Get all metrics from Kube-apiserver
+[[inputs.prometheus]]
+  # An array of urls to scrape metrics from.
+  urls = ["http://my-kube-apiserver:8080/metrics"]
+```
+
+You can use more complex configuration
+to filter and some tags
+
+```toml
+# Get all metrics from Kube-apiserver
+[[inputs.prometheus]]
+  # An array of urls to scrape metrics from.
+  urls = ["http://my-kube-apiserver:8080/metrics"]
+  # Get only metrics with "apiserver_" string is in metric name
+  namepass = ["apiserver_"]
+  # Add a metric name prefix
+  name_prefix = "k8s_"
+  # Add tags to be able to make beautiful dashboards
+  [inputs.prometheus.tags]
+    kubeservice = "kube-apiserver"
+```
+
+### Measurements & Fields & Tags:
+
+Measurements and fields could be any thing.
+It just depends of what you're quering.
+
+Example:
+
+```
+# HELP go_gc_duration_seconds A summary of the GC invocation durations.
+# TYPE go_gc_duration_seconds summary
+go_gc_duration_seconds{quantile="0"} 0.00010425500000000001
+go_gc_duration_seconds{quantile="0.25"} 0.000139108
+go_gc_duration_seconds{quantile="0.5"} 0.00015749400000000002
+go_gc_duration_seconds{quantile="0.75"} 0.000331463
+go_gc_duration_seconds{quantile="1"} 0.000667154
+go_gc_duration_seconds_sum 0.0018183950000000002
+go_gc_duration_seconds_count 7
+# HELP go_goroutines Number of goroutines that currently exist.
+# TYPE go_goroutines gauge
+go_goroutines 15
+```
+
+- go_goroutines,
+    - gauge (integer, unit)
+- go_gc_duration_seconds
+    - field3 (integer, bytes)
+
+- All measurements have the following tags:
+    - url=http://my-kube-apiserver:8080/metrics
+- go_goroutines has the following tags:
+    - kubeservice=kube-apiserver
+- go_gc_duration_seconds has the following tags:
+    - kubeservice=kube-apiserver
+
+### Example Output:
+
+Example of output with configuration given above:
+
+```
+$ ./telegraf -config telegraf.conf  -test
+k8s_go_goroutines,kubeservice=kube-apiserver,url=http://my-kube-apiserver:8080/metrics gauge=536 1456857329391929813
+k8s_go_gc_duration_seconds,kubeservice=kube-apiserver,url=http://my-kube-apiserver:8080/metrics 0=0.038002142,0.25=0.041732467,0.5=0.04336492,0.75=0.047271799,1=0.058295811,count=0,sum=208.334617406 1456857329391929813
+```