[receiver/prometheus] Add fallback_scrape_protocol when using target allocator (open-telemetry#39672)

This applies a similar fix as 72b834b,
when using TargetAllocator.

#### Description
For the same reason as we already add a `fallback_scrape_protocol` to
the config, this PR add the same behavior when using the Target
Allocator.

It's important for me, as even the `fallbackScrapeProtocol` from the
Prometheus CRD is ignored (I guess it's because the allocator advertise
itself as version 2.55.1, which does not support (nor require)
`fallback_scrape_protocol`).

Maybe it's a change that should be done on the target-allocator side,
but I kind of like the symmetry with what we're already doing with
provided configuration.

#### Link to tracking issue
Same issue as open-telemetry#37902, but when using a target allocator

---------

Signed-off-by: MATILLAT Quentin <[email protected]>
Co-authored-by: George Krajcsovits <[email protected]>
Co-authored-by: Arthur Silva Sens <[email protected]>

Author: 3 people

.chloggen/prom-scrape-config-ta.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: prometheusreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add 'fallback_scrape_protocol' to entries obtained through Target Allocator, unless already defined.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [39672]
+
+# (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: This applies the same configuration change that is already applied on static configuration.
+
+# 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: [user]

receiver/prometheusreceiver/targetallocator/manager.go

@@ -150,6 +150,10 @@ func (m *Manager) sync(compareHash uint64, httpClient *http.Client) (uint64, err
 			scrapeConfig.HTTPClientConfig = commonconfig.HTTPClientConfig(*m.cfg.HTTPScrapeConfig)
 		}
 
+		if scrapeConfig.ScrapeFallbackProtocol == "" {
+			scrapeConfig.ScrapeFallbackProtocol = promconfig.PrometheusText0_0_4
+		}
+
 		m.promCfg.ScrapeConfigs = append(m.promCfg.ScrapeConfigs, scrapeConfig)
 	}
 

receiver/prometheusreceiver/targetallocator/manager_test.go

@@ -67,9 +67,10 @@ type expectedMetricRelabelConfigTestResult struct {
 }
 
 type expectedTestResultJobMap struct {
-	Targets             []string
-	Labels              model.LabelSet
-	MetricRelabelConfig *expectedMetricRelabelConfigTestResult
+	Targets                []string
+	Labels                 model.LabelSet
+	MetricRelabelConfig    *expectedMetricRelabelConfigTestResult
+	ScrapeFallbackProtocol promconfig.ScrapeProtocol
 }
 
 type expectedTestResult struct {
@@ -284,14 +285,15 @@ func TestTargetAllocatorJobRetrieval(t *testing.T) {
 					"/scrape_configs": {
 						mockTargetAllocatorResponseRaw{code: 200, data: map[string]map[string]any{
 							"job1": {
-								"job_name":               "job1",
-								"scrape_interval":        "30s",
-								"scrape_timeout":         "30s",
-								"scrape_protocols":       []string{"OpenMetricsText1.0.0", "OpenMetricsText0.0.1", "PrometheusText0.0.4"},
-								"metrics_path":           "/metrics",
-								"scheme":                 "http",
-								"relabel_configs":        nil,
-								"metric_relabel_configs": nil,
+								"job_name":                 "job1",
+								"scrape_interval":          "30s",
+								"scrape_timeout":           "30s",
+								"scrape_protocols":         []string{"OpenMetricsText1.0.0", "OpenMetricsText0.0.1", "PrometheusText0.0.4"},
+								"metrics_path":             "/metrics",
+								"scheme":                   "http",
+								"relabel_configs":          nil,
+								"metric_relabel_configs":   nil,
+								"fallback_scrape_protocol": promconfig.PrometheusText1_0_0,
 							},
 							"job2": {
 								"job_name":               "job2",
@@ -369,13 +371,15 @@ func TestTargetAllocatorJobRetrieval(t *testing.T) {
 							"__meta_datacenter":     "london",
 							"__meta_prometheus_job": "node",
 						},
+						ScrapeFallbackProtocol: promconfig.PrometheusText1_0_0,
 					},
 					"job2": {
 						Targets: []string{"10.0.40.2:9100", "10.0.40.3:9100"},
 						Labels: map[model.LabelName]model.LabelValue{
 							"__meta_datacenter":     "london",
 							"__meta_prometheus_job": "alertmanager",
 						},
+						ScrapeFallbackProtocol: promconfig.PrometheusText0_0_4, // Tests default value
 					},
 				},
 			},
@@ -778,6 +782,15 @@ func TestTargetAllocatorJobRetrieval(t *testing.T) {
 								}
 							}
 						}
+
+						if s.ScrapeFallbackProtocol != "" {
+							for _, sc := range manager.promCfg.ScrapeConfigs {
+								if sc.JobName == job {
+									require.Equal(t, sc.ScrapeFallbackProtocol, s.ScrapeFallbackProtocol)
+								}
+							}
+						}
+
 						found = true
 					}
 					require.True(t, found, "Returned job is not defined in expected values", group)