fix: content-type header for metrics endpoints

    Issue #13:
    - JSON endpoints now return the correct Content-Type: `application/json; charset=utf-8`.

    Additionally:
    - Added tests to verify the correctness of the Content-Type header.

    This fix ensures proper header formatting and alignment with client expectations.

Author: vakhov

Diff for: .gitignore

@@ -10,3 +10,4 @@ tags
 doc/apidoc/
 .idea
 *.swp
+.DS_Store

Diff for: roles/metrics-export.lua

@@ -100,7 +100,17 @@ end
 local http_handlers = {
     json = function(req)
         local json_exporter = require('metrics.plugins.json')
-        return req:render({ text = json_exporter.export() })
+        local json_data = json_exporter.export()
+
+        local content_type = 'application/json'
+
+        if req.httpd.options.charset then
+            content_type = string.format('%s; charset=%s', content_type, req.httpd.options.charset)
+        end
+
+        local response = req:render({ text = json_data })
+        response.headers['content-type'] = content_type
+        return response
     end,
     prometheus = function(...)
         local http_handler = require('metrics.plugins.prometheus').collect_http

Diff for: test/unit/http_test.lua

@@ -55,8 +55,16 @@ local function assert_json(uri, tls_opts)
     local response = http_client:get(uri, tls_opts)
     t.assert(response.body)
 
-    local data = response.body
-    local decoded = json.decode(data)
+    local decoded = json.decode(response.body)
+    if type(decoded) == 'string' then
+        decoded = json.decode(decoded)
+    end
+    t.assert(#decoded > 0)
+    if type(decoded) ~= 'table' then
+        print('Raw response body:', response.body)
+    end
+    t.assert(type(decoded) == 'table', 'Response body is not a valid JSON object')
+
     for _, node in ipairs(decoded) do
         node.timestamp = nil
     end
@@ -1140,3 +1148,53 @@ for name, case in pairs(test_tls_cases) do
         end
     end
 end
+
+local function assert_content_type(uri, expected_content_type, tls_opts)
+    local response = http_client:get(uri, tls_opts)
+    t.assert_equals(response.status, 200)
+    t.assert_equals(response.headers['content-type'], expected_content_type)
+end
+
+local test_content_type_cases = {
+    ['json'] = {
+        cfg = {
+            http = {
+                {
+                    listen = 8081,
+                    endpoints = {
+                        {
+                            path = "/json_metrics",
+                            format = "json",
+                        },
+                    },
+                },
+            },
+        },
+        expected_url = "http://127.0.0.1:8081/json_metrics",
+        expected_content_type = "application/json; charset=utf-8",
+    },
+    ['prometheus'] = {
+        cfg = {
+            http = {
+                {
+                    listen = 8081,
+                    endpoints = {
+                        {
+                            path = "/prometheus_metrics",
+                            format = "prometheus",
+                        },
+                    },
+                },
+            },
+        },
+        expected_url = "http://127.0.0.1:8081/prometheus_metrics",
+        expected_content_type = "text/plain; charset=utf8",
+    },
+}
+
+for name, case in pairs(test_content_type_cases) do
+    g['test_content_type_' .. name] = function(cg)
+        cg.role.apply(case.cfg)
+        assert_content_type(case.expected_url, case.expected_content_type)
+    end
+end