Expose internal metrics at default localhost:8888 address (#5706)

Expose internal metrics at default `localhost:8888` address instead of `${SPLUNK_LISTEN_INTERFACE}:8888`

This can be changed by changing default values in `service::telemetry::metrics::section`:
```yaml
service:
  telemetry:
    metrics:
      readers:
        - pull:
            exporter:
              prometheus:
                host: localhost
                port: 8888
```
This also removes a warning about deprecated `service::telemetry::metrics::address`.

Author: dmitryax

CHANGELOG.md

@@ -6,6 +6,20 @@
 
 - (Splunk) Automatic Discovery:
   - Switch bundled NGINX discovery to create [OpenTelemetry NGINX receiver](https://docs.splunk.com/observability/en/gdi/opentelemetry/components/nginx-receiver.html#nginx-receiver) instead of the Smart Agent NGINX monitor ([#5689](https://github.com/signalfx/splunk-otel-collector/pull/5689))
+- (Splunk) Expose internal metrics at default `localhost:8888` address instead of `${SPLUNK_LISTEN_INTERFACE}:8888` ([#5706](https://github.com/signalfx/splunk-otel-collector/pull/5706))
+  This can be changed in `service::telemetry::metrics` section:  
+  ```yaml
+  service:
+    telemetry:
+      metrics:
+        readers:
+          - pull:
+              exporter:
+                prometheus:
+                  host: localhost
+                  port: 8888
+  ```
+  This also removes a warning about deprecated `service::telemetry::metrics::address`.
 
 ### 🚩Deprecations 🚩
 

cmd/otelcol/config/collector/agent_config.yaml

@@ -71,7 +71,7 @@ receivers:
       - job_name: 'otel-collector'
         scrape_interval: 10s
         static_configs:
-        - targets: ["${SPLUNK_LISTEN_INTERFACE}:8888"]
+        - targets: ["0.0.0.0:8888"]
         metric_relabel_configs:
           - source_labels: [ __name__ ]
             regex: 'otelcol_rpc_.*'
@@ -166,9 +166,6 @@ exporters:
     verbosity: detailed
 
 service:
-  telemetry:
-    metrics:
-      address: "${SPLUNK_LISTEN_INTERFACE}:8888"
   extensions: [health_check, http_forwarder, zpages, smartagent]
   pipelines:
     traces:

cmd/otelcol/config/collector/gateway_config.yaml

@@ -37,7 +37,7 @@ receivers:
       - job_name: 'otel-collector'
         scrape_interval: 10s
         static_configs:
-        - targets: ['${SPLUNK_LISTEN_INTERFACE}:8888']
+        - targets: ['0.0.0.0:8888']
         metric_relabel_configs:
           - source_labels: [ __name__ ]
             regex: 'otelcol_rpc_.*'
@@ -127,9 +127,6 @@ exporters:
     log_data_enabled: false
 
 service:
-  telemetry:
-    metrics:
-      address: "${SPLUNK_LISTEN_INTERFACE}:8888"
   extensions: [health_check, http_forwarder, zpages]
   pipelines:
     traces:

cmd/otelcol/fips/config/agent_config.yaml

@@ -65,7 +65,7 @@ receivers:
       - job_name: 'otel-collector'
         scrape_interval: 10s
         static_configs:
-        - targets: ["${SPLUNK_LISTEN_INTERFACE}:8888"]
+        - targets: ["0.0.0.0:8888"]
         metric_relabel_configs:
           - source_labels: [ __name__ ]
             regex: 'otelcol_rpc_.*'
@@ -156,9 +156,6 @@ exporters:
     verbosity: detailed
 
 service:
-  telemetry:
-    metrics:
-      address: "${SPLUNK_LISTEN_INTERFACE}:8888"
   extensions: [health_check, http_forwarder, zpages]
   pipelines:
     traces:

cmd/otelcol/fips/config/gateway_config.yaml

@@ -37,7 +37,7 @@ receivers:
       - job_name: 'otel-collector'
         scrape_interval: 10s
         static_configs:
-        - targets: ['${SPLUNK_LISTEN_INTERFACE}:8888']
+        - targets: ['0.0.0.0:8888']
         metric_relabel_configs:
           - source_labels: [ __name__ ]
             regex: 'otelcol_rpc_.*'

tests/general/default_config_test.go

@@ -159,7 +159,7 @@ func TestDefaultGatewayConfig(t *testing.T) {
 									"scrape_interval": "10s",
 									"static_configs": []any{
 										map[string]any{
-											"targets": []any{fmt.Sprintf("%s:8888", ip)},
+											"targets": []any{"0.0.0.0:8888"},
 										},
 									},
 								},
@@ -177,7 +177,6 @@ func TestDefaultGatewayConfig(t *testing.T) {
 					},
 				},
 				"service": map[string]any{
-					"telemetry":  map[string]any{"metrics": map[string]any{"address": fmt.Sprintf("%s:8888", ip)}},
 					"extensions": []any{"health_check", "http_forwarder", "zpages"},
 					"pipelines": map[string]any{
 						"logs": map[string]any{
@@ -369,7 +368,7 @@ func TestDefaultAgentConfig(t *testing.T) {
 									"scrape_interval": "10s",
 									"static_configs": []any{
 										map[string]any{
-											"targets": []any{fmt.Sprintf("%s:8888", ip)},
+											"targets": []any{"0.0.0.0:8888"},
 										},
 									},
 								},
@@ -382,7 +381,6 @@ func TestDefaultAgentConfig(t *testing.T) {
 					"nop":                    nil,
 				},
 				"service": map[string]any{
-					"telemetry":  map[string]any{"metrics": map[string]any{"address": fmt.Sprintf("%s:8888", ip)}},
 					"extensions": []any{"health_check", "http_forwarder", "zpages", "smartagent"},
 					"pipelines": map[string]any{
 						"logs": map[string]any{