metrics: Fix hidden panic while observing with bad exemplars (#3733)

* metrics: Fixing panic while observing with bad exemplars

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

* Minor cleanup

The server is already added to the context. So, we can simply use that
to get the server name, which is a field on the server.

* Add integration test for auto HTTP->HTTPS redirects

A test like this would have caught the problem in the first place

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

1 files changed, 16 insertions, 1 deletions

diff --git a/modules/caddyhttp/metrics_test.go b/modules/caddyhttp/metrics_test.go
index 14248a3..6ac591f 100644
--- a/modules/caddyhttp/metrics_test.go
+++ b/modules/caddyhttp/metrics_test.go
@@ -1,6 +1,7 @@
 package caddyhttp
 
 import (
+	"context"
 	"errors"
 	"net/http"
 	"net/http/httptest"
@@ -9,6 +10,20 @@ import (
 	"github.com/prometheus/client_golang/prometheus/testutil"
 )
 
+func TestServerNameFromContext(t *testing.T) {
+	ctx := context.Background()
+	expected := "UNKNOWN"
+	if actual := serverNameFromContext(ctx); actual != expected {
+		t.Errorf("Not equal: expected %q, but got %q", expected, actual)
+	}
+
+	in := "foo"
+	ctx = context.WithValue(ctx, ServerCtxKey, &Server{name: in})
+	if actual := serverNameFromContext(ctx); actual != in {
+		t.Errorf("Not equal: expected %q, but got %q", in, actual)
+	}
+}
+
 func TestMetricsInstrumentedHandler(t *testing.T) {
 	handlerErr := errors.New("oh noes")
 	response := []byte("hello world!")
@@ -26,7 +41,7 @@ func TestMetricsInstrumentedHandler(t *testing.T) {
 		return h.ServeHTTP(w, r)
 	})
 
-	ih := newMetricsInstrumentedHandler("foo", "bar", mh)
+	ih := newMetricsInstrumentedHandler("bar", mh)
 
 	r := httptest.NewRequest("GET", "/", nil)
 	w := httptest.NewRecorder()