[receiver/datadog] fix feature discovery (open-telemetry#34726)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->

**Link to tracking Issue:** 
Fix
open-telemetry#34718

**Testing:** 
this otel config

```
receivers:
  datadog:
    endpoint: 0.0.0.0:8080
    read_timeout: 60s

processors:
  probabilistic_sampler:
    sampling_percentage: 0

exporters:
  debug:
    verbosity: detailed

service:
  pipelines:
    traces:
      receivers: [datadog]
      processors: [probabilistic_sampler]
      exporters: [debug] 
```
tcpdump on 8080 port and having this http request performed : 
```
PUT /v0.4/traces HTTP/1.1
Content-Type: application/msgpack
Content-Length: 0
Host: 10.0.0.2:8080
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/3.12.15

HTTP/1.1 400 Bad Request
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Sun, 18 Aug 2024 22:06:39 GMT
Content-Length: 23

Fake featuresdiscovery
```
**Documentation:** <Describe the documentation added.>

.chloggen/datadogreceiver-featurediscovery.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: datadogreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: add feature discovery
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [34718]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/datadogreceiver/receiver.go

@@ -109,6 +109,10 @@ func (ddr *datadogReceiver) Shutdown(ctx context.Context) (err error) {
 }
 
 func (ddr *datadogReceiver) handleTraces(w http.ResponseWriter, req *http.Request) {
+	if req.ContentLength == 0 { // Ping mecanism of Datadog SDK perform http request with empty body when GET /info not implemented.
+		http.Error(w, "Fake featuresdiscovery", http.StatusBadRequest) // The response code should be different of 404 to be considered ok by Datadog SDK.
+		return
+	}
 	obsCtx := ddr.tReceiver.StartTracesOp(req.Context())
 	var err error
 	var spanCount int

receiver/datadogreceiver/receiver_test.go

@@ -84,6 +84,12 @@ func TestDatadogServer(t *testing.T) {
 			expectCode:    http.StatusBadRequest,
 			expectContent: "Unable to unmarshal reqs\n",
 		},
+		{
+			name:          "Fake featuresdiscovery",
+			op:            nil, // Content-length: 0.
+			expectCode:    http.StatusBadRequest,
+			expectContent: "Fake featuresdiscovery\n",
+		},
 	} {
 		tc := tc
 		t.Run(tc.name, func(t *testing.T) {