Throw an error when http_middelware is processing a wrong handler

CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Removed
 - Average collector
 
+### Fixed
+- Throw an error when http_middelware is processing a wrong handler [#199](https://github.com/tarantool/metrics/issues/199)
+
 ## [0.10.0] - 2021-08-03
 ### Changed
 - metrics registry refactoring to search with `O(1)` [#188](https://github.com/tarantool/metrics/issues/188)

metrics/http_middleware.lua

@@ -71,6 +71,10 @@ end
 -- ... arguments for pcall to instrument
 function export.observe(collector, route, ...)
     return collector:observe_latency(function(ok, result)
+        if type(result) ~= 'table' then
+            error(('incorrect http handler for %s %s: expecting return response object'):
+                format(route.method, route.path), 0)
+        end
         return {
             path = route.path,
             method = route.method,

test/http_middleware_test.lua

@@ -148,6 +148,20 @@ g.test_v1_middleware = function()
     )
 end
 
+g.test_v1_wrong_handler = function()
+    local request = {endpoint = table.copy(route)}
+
+    local observer = stub_observer(function() end)
+    local handler = function()
+        -- we forget to return result!
+        -- return result
+    end
+    t.assert_error_msg_contains(
+        'incorrect http handler for POST /some/path: expecting return response object',
+        http_middleware.v1(handler, observer), request
+    )
+end
+
 g.test_v2_middleware = function()
     local httpd = require('http.server').new('127.0.0.1', 12345)
     t.skip_if(httpd.set_router == nil, 'Skip http 2.x test')
@@ -170,3 +184,24 @@ g.test_v2_middleware = function()
         'number'
     )
 end
+
+g.test_v2_wrong_handler = function()
+    local httpd = require('http.server').new('127.0.0.1', 12345)
+    t.skip_if(httpd.set_router == nil, 'Skip http 2.x test')
+    local router = require('http.router').new()
+    router:route(route, function()
+        -- we forget to return result!
+        -- return result
+    end)
+    router:use(http_middleware.v2(), {name = 'http_instrumentation'})
+    httpd:set_router(router)
+    httpd:start()
+    local client = require('http.client').new()
+    local response = client:request(route.method, 'http://127.0.0.1:12345' .. route.path)
+    httpd:stop()
+    t.assert_equals(response.status, 500)
+    t.assert_str_contains(
+        response.body,
+        'incorrect http handler for POST /some/path: expecting return response object'
+    )
+end