Update 'Metrics'

Metrics.md

@@ -0,0 +1,45 @@
+All metrics are prefixed with the `pages` namespace. This makes it easy to narrow down to the specific metrics for the
+system.
+
+## pages_page_view_count
+
+The number of page views for a given path and their associated referrer.
+
+```text
+# HELP pages_page_view_count the number of times a given page has been viewed and by what referrer
+# TYPE pages_page_view_count counter
+pages_page_view_count{country="",path="/charts/",referrer="http://localhost:8080/blog/"} 1
+```
+
+## pages_page_session_seconds
+
+Histogram of how long users spend on a page.
+
+```text
+# HELP pages_page_session_seconds how long someone spent on a given page
+# TYPE pages_page_session_seconds histogram
+pages_page_session_seconds_bucket{country="",path="/",le="0.005"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="0.01"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="0.025"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="0.05"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="0.1"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="0.25"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="0.5"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="1"} 0
+pages_page_session_seconds_bucket{country="",path="/",le="2.5"} 1
+pages_page_session_seconds_bucket{country="",path="/",le="5"} 1
+pages_page_session_seconds_bucket{country="",path="/",le="10"} 1
+pages_page_session_seconds_bucket{country="",path="/",le="+Inf"} 1
+pages_page_session_seconds_sum{country="",path="/"} 1.855976549
+pages_page_session_seconds_count{country="",path="/"} 1
+```
+
+## pages_page_sessions_active
+
+A gauge that approximates the current number of sessions per page.
+
+```text
+# HELP pages_page_sessions_active the number of current sessions for a given page
+# TYPE pages_page_sessions_active gauge
+pages_page_sessions_active{country="",path="/"} 1
+```