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>

modules/caddyhttp/server.go

@@ -122,6 +122,8 @@ type Server struct {
 	// ⚠️ Experimental feature; subject to change or removal.
 	AllowH2C bool `json:"allow_h2c,omitempty"`
 
+	name string
+
 	primaryHandlerChain Handler
 	errorHandlerChain   Handler
 	listenerWrappers    []caddy.ListenerWrapper