diagnostics: Implemented collection functions and create first metrics

- Also implemented robust error handling and failovers
- Vendored klauspost/cpuid

caddyhttp/httpserver/plugin.go

@@ -29,6 +29,7 @@ import (
 	"github.com/mholt/caddy/caddyfile"
 	"github.com/mholt/caddy/caddyhttp/staticfiles"
 	"github.com/mholt/caddy/caddytls"
+	"github.com/mholt/caddy/diagnostics"
 )
 
 const serverType = "http"
@@ -205,6 +206,8 @@ func (h *httpContext) MakeServers() ([]caddy.Server, error) {
 		}
 	}
 
+	diagnostics.Set("num_sites", len(h.siteConfigs))
+
 	// we must map (group) each config to a bind address
 	groups, err := groupSiteConfigsByListenAddr(h.siteConfigs)
 	if err != nil {

caddyhttp/httpserver/server.go

@@ -36,6 +36,7 @@ import (
 	"github.com/mholt/caddy"
 	"github.com/mholt/caddy/caddyhttp/staticfiles"
 	"github.com/mholt/caddy/caddytls"
+	"github.com/mholt/caddy/diagnostics"
 )
 
 // Server is the HTTP server implementation.
@@ -345,6 +346,8 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}()
 
+	go diagnostics.AppendUniqueString("user_agent", r.Header.Get("User-Agent"))
+
 	// copy the original, unchanged URL into the context
 	// so it can be referenced by middlewares
 	urlCopy := *r.URL