[telemetrygen] Fix case where root CAs should inherit from host envir…

…onment (open-telemetry#31250)

**Description:** 

Adding TLS configuration to telemetrygen made it so that providing root
CA information is mandatory. This is for the case when you are
generating CAs and using them to sign certificates. In the case where
the CA that signed the certificate is a known trusted CA, we shouldn't
have to provide its CA certificate. This PR fixes this problem by
allowing the code to pull its CA pool from the host environment when CAs
are not explicitly supplied.

**Link to tracking Issue:**
[github.com/open-telemetry/issues/31191](open-telemetry#31191)

**Testing:** Generate metrics, logs, traces against http and grpc
endpoints with legitimate certificates

**Documentation:** <Describe the documentation added.>

.chloggen/dev_fix-broken-root-ca.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: cmd/telemetrygen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Inherit root CAs from the host environment if not supplied on the command line.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [31191]
+
+# (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: []

cmd/telemetrygen/internal/common/tls_utils.go

@@ -35,9 +35,15 @@ func GetTLSCredentialsForGRPCExporter(caFile string, cAuth ClientAuth) (credenti
 		return nil, err
 	}
 
-	creds := credentials.NewTLS(&tls.Config{
-		RootCAs: pool,
-	})
+	var creds credentials.TransportCredentials
+
+	if caFile != "" {
+		creds = credentials.NewTLS(&tls.Config{
+			RootCAs: pool,
+		})
+	} else {
+		creds = credentials.NewTLS(&tls.Config{})
+	}
 
 	// Configuration for mTLS
 	if cAuth.Enabled {
@@ -60,8 +66,14 @@ func GetTLSCredentialsForHTTPExporter(caFile string, cAuth ClientAuth) (*tls.Con
 		return nil, err
 	}
 
-	tlsCfg := tls.Config{
-		RootCAs: pool,
+	var tlsCfg tls.Config
+
+	if caFile != "" {
+		tlsCfg = tls.Config{
+			RootCAs: pool,
+		}
+	} else {
+		tlsCfg = tls.Config{}
 	}
 
 	// Configuration for mTLS