Add parameter to optionally strip deploy path. (#639)

* Add parameter to optionally strip deploy path.
Closes #631.

Signed-off-by: Lapo Luchini <lapo@lapo.it>

* Add documentation and test as requested in PR#639.

Signed-off-by: Lapo Luchini <lapo@lapo.it>

Author: lapo-luchini

README.md

@@ -577,7 +577,10 @@ measuring is also configurable, via the `path-components` init parameter. By
 default, the servlet filter will record each path differently, but by setting an
 integer here, you can tell the filter to only record up to the Nth slashes. That
 is, all requests with greater than N "/" characters in the servlet URI path will
-be measured in the same bucket and you will lose that granularity.
+be measured in the same bucket and you will lose that granularity. The init
+parameter `strip-context-path` can be used to strip the leading part of the URL
+which is part of the deploy context (i.e. the folder the servlet is deployed to),
+so that the same servlet deployed to different paths can lead to similar metrics.
 
 The code below is an example of the XML configuration for the filter. You will
 need to place this (replace your own values) code in your
@@ -605,6 +608,11 @@ need to place this (replace your own values) code in your
     <param-name>path-components</param-name>
     <param-value>1</param-value>
   </init-param>
+  <!-- strip-context-path is optional, defaults to false -->
+  <init-param>
+    <param-name>strip-context-path</param-name>
+    <param-value>false</param-value>
+  </init-param>
 </filter>
 
 <!-- You will most likely want this to be the first filter in the chain

simpleclient_servlet/src/main/java/io/prometheus/client/filter/MetricsFilter.java

@@ -25,6 +25,10 @@
  * (including servlet context path), but can be configured with the {@code path-components} init parameter. Any number
  * provided that is less than 1 will provide the full path granularity (warning, this may affect performance).
  *
+ * <p>The {@code strip-context-path} init parameter can be used to avoid including the leading path components which are
+ * part of the context (i.e. the folder where the servlet is deployed) so that the same project deployed under different
+ * paths can produce the same metrics.
+ *
  * <p>The Histogram buckets can be configured with a {@code buckets} init parameter whose value is a comma-separated list
  * of valid {@code double} values.
  *
@@ -50,6 +54,11 @@
  *      <param-name>path-components</param-name>
  *      <param-value>0</param-value>
  *   </init-param>
+ *   <!-- strip-context-path is optional, defaults to false -->
+ *   <init-param>
+ *      <param-name>strip-context-path</param-name>
+ *      <param-value>false</param-value>
+ *   </init-param>
  * </filter>
  * }</pre>
  *
@@ -60,6 +69,7 @@ public class MetricsFilter implements Filter {
     static final String HELP_PARAM = "help";
     static final String METRIC_NAME_PARAM = "metric-name";
     static final String BUCKET_CONFIG_PARAM = "buckets";
+    static final String STRIP_CONTEXT_PATH_PARAM = "strip-context-path";
     static final String UNKNOWN_HTTP_STATUS_CODE = "";
 
     private Histogram histogram = null;
@@ -68,6 +78,7 @@ public class MetricsFilter implements Filter {
     // Package-level for testing purposes.
     int pathComponents = 1;
     private String metricName = null;
+    boolean stripContextPath = false;
     private String help = "The time taken fulfilling servlet requests";
     private double[] buckets = null;
 
@@ -145,6 +156,10 @@ public void init(FilterConfig filterConfig) throws ServletException {
                     buckets[i] = Double.parseDouble(bucketParams[i]);
                 }
             }
+
+            if (!isEmpty(filterConfig.getInitParameter(STRIP_CONTEXT_PATH_PARAM))) {
+                stripContextPath = Boolean.parseBoolean(filterConfig.getInitParameter(STRIP_CONTEXT_PATH_PARAM));
+            }
         }
 
         if (buckets != null) {
@@ -171,6 +186,9 @@ public void doFilter(ServletRequest servletRequest, ServletResponse servletRespo
         HttpServletRequest request = (HttpServletRequest) servletRequest;
 
         String path = request.getRequestURI();
+        if (stripContextPath) {
+            path = path.substring(request.getContextPath().length());
+        }
 
         String components = getComponents(path);
         String method = request.getMethod();

simpleclient_servlet/src/test/java/io/prometheus/client/filter/MetricsFilterTest.java

@@ -17,6 +17,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.doAnswer;
@@ -231,4 +232,26 @@ public void testStatusCodeWithNonHttpServletResponse() throws Exception {
         assertNotNull(sampleValue);
         assertEquals(1, sampleValue, 0.0001);
     }
+
+    @Test
+    public void testStripContextPath() throws Exception {
+        String metricName = "foo";
+        FilterConfig cfg = mock(FilterConfig.class);
+        when(cfg.getInitParameter(MetricsFilter.METRIC_NAME_PARAM)).thenReturn(metricName);
+        when(cfg.getInitParameter(MetricsFilter.PATH_COMPONENT_PARAM)).thenReturn("0");
+        when(cfg.getInitParameter(MetricsFilter.STRIP_CONTEXT_PATH_PARAM)).thenReturn("true");
+        f.init(cfg);
+        assertTrue(f.stripContextPath);
+        HttpServletRequest req = mock(HttpServletRequest.class);
+        when(req.getRequestURI()).thenReturn("/foo/bar/baz/bang");
+        when(req.getContextPath()).thenReturn("/foo/bar");
+        when(req.getMethod()).thenReturn(HttpMethods.GET);
+        HttpServletResponse res = mock(HttpServletResponse.class);
+        FilterChain c = mock(FilterChain.class);
+        f.doFilter(req, res, c);
+        verify(c).doFilter(req, res);
+        final Double sampleValue = CollectorRegistry.defaultRegistry.getSampleValue(metricName + "_count", new String[]{"path", "method"}, new String[]{"/baz/bang", HttpMethods.GET});
+        assertNotNull(sampleValue);
+        assertEquals(1, sampleValue, 0.0001);
+    }
 }