fix(tracing): ensure sampling decisions apply to correct trace ID

Kong generates a temporary trace ID when spans are created, this trace
ID can be overwritten if tracing headers are passed in the request.
Before this change, the global sampling rate from kong.conf was applied
to the temporary trace ID, which was only valid for traces created by
Kong, and not in distributed tracing scenarios.

This commit ensures the sampling decision is only taken once, after
headers have been read, when the trace id is final. This ensures Kong's
sampling decision matches that of other tracing systems and therefore
guarantees that traces are not broken when sampling rates are set to
the same values.

changelog/unreleased/kong/fix-tracing-sampling-rate.yml

@@ -0,0 +1,3 @@
+message: "**OpenTelemetry:** improved accuracy of sampling decisions."
+type: bugfix
+scope: Plugin

kong/pdk/tracing.lua

@@ -575,12 +575,13 @@ local function new_tracer(name, options)
   -- @tparam number sampling_rate the sampling rate to apply for the
   -- probability sampler
   -- @treturn bool sampled value of sampled for this trace
-  function self:get_sampling_decision(parent_should_sample, sampling_rate)
+  function self:get_sampling_decision(parent_should_sample, plugin_sampling_rate)
     local ctx = ngx.ctx
 
     local sampled
     local root_span = ctx.KONG_SPANS and ctx.KONG_SPANS[1]
     local trace_id = tracing_context.get_raw_trace_id(ctx)
+    local sampling_rate = plugin_sampling_rate or kong.configuration.tracing_sampling_rate
 
     if not root_span or root_span.attributes["kong.propagation_only"] then
       -- should not sample if there is no root span or if the root span is
@@ -592,12 +593,7 @@ local function new_tracer(name, options)
       -- and Kong is configured to only do headers propagation
       sampled = parent_should_sample
 
-    elseif not sampling_rate then
-      -- no custom sampling_rate was passed:
-      -- reuse the sampling result of the root_span
-      sampled = root_span.should_sample == true
-
-    else
+    elseif sampling_rate then
       -- use probability-based sampler
       local err
       sampled, err = self.sampler(trace_id, sampling_rate)

spec/03-plugins/37-opentelemetry/04-exporter_spec.lua

@@ -243,6 +243,80 @@ for _, strategy in helpers.each_strategy() do
       end)
     end
 
+
+    describe("With config.sampling_rate unset, using global sampling rate: 0.5", function ()
+      local mock
+      local sampling_rate = 0.5
+       -- this trace_id is always sampled with 0.5 rate
+      local sampled_trace_id = "92a54b3e1a7c4f2da9e44b8a6f3e1dab"
+       -- this trace_id is never sampled with 0.5 rate
+      local non_sampled_trace_id = "4bf92f3577b34da6a3ce929d0e0e4736"
+
+      lazy_setup(function()
+        bp, _ = assert(helpers.get_db_utils(strategy, {
+          "services",
+          "routes",
+          "plugins",
+        }, { "opentelemetry" }))
+
+        setup_instrumentations("all", {}, nil, nil, nil, nil, sampling_rate)
+        mock = helpers.http_mock(HTTP_SERVER_PORT, { timeout = HTTP_MOCK_TIMEOUT })
+      end)
+
+      lazy_teardown(function()
+        helpers.stop_kong()
+        if mock then
+          mock("close", true)
+        end
+      end)
+
+      it("does not sample spans when trace_id == non_sampled_trace_id", function()
+        local cli = helpers.proxy_client(7000, PROXY_PORT)
+        local r = assert(cli:send {
+          method  = "GET",
+          path    = "/",
+          headers = {
+            traceparent = "00-" .. non_sampled_trace_id .. "-0123456789abcdef-01"
+          }
+        })
+        assert.res_status(200, r)
+
+        cli:close()
+
+        ngx.sleep(2)
+        local lines = mock()
+        assert.is_falsy(lines)
+      end)
+
+      it("samples spans when trace_id == sampled_trace_id", function ()
+        for _ = 1, 10 do
+          local body
+          helpers.wait_until(function()
+            local cli = helpers.proxy_client(7000, PROXY_PORT)
+            local r = assert(cli:send {
+              method  = "GET",
+              path    = "/",
+              headers = {
+                traceparent = "00-" .. sampled_trace_id .. "-0123456789abcdef-01"
+              }
+            })
+            assert.res_status(200, r)
+
+            cli:close()
+
+            local lines
+            lines, body = mock()
+            return lines
+          end, 10)
+
+          local decoded = assert(pb.decode("opentelemetry.proto.collector.trace.v1.ExportTraceServiceRequest", body))
+          assert.not_nil(decoded)
+          local scope_spans = decoded.resource_spans[1].scope_spans
+          assert.is_true(#scope_spans > 0, scope_spans)
+        end
+      end)
+    end)
+
     for _, case in ipairs{
       {true, true, true},
       {true, true, nil},