Bake in pprof data from production to optimise hot paths

This data was collected by setting `COLLECT_METRICS=yes` on production and then running:

```bash
$ while true; do curl http://localhost:6060/debug/pprof/profile?seconds=30 --output profiles/$(date +%s).pprof; sleep 270; done
```

To gather a chunk of metrics every 5 mins for 30 seconds for a couple of hours. The traffic to the server came in bursts, but included lots of visits to jupyter- which is massively asset heavy and makes loads of calls to /api/v1/auth.

At some point we can start thinking about gathering data from loads of instances over many days with lots of requests; at that point this commit will be useful prep

.gitignore

@@ -1 +1,2 @@
 littleauth
+profiles/*.pprof

Makefile

@@ -1,4 +1,4 @@
-go_build_cmd ?= CGO_ENABLED=1 go build -ldflags="-s -w" -trimpath -race
+go_build_cmd ?= CGO_ENABLED=1 go build -ldflags="-s -w" -trimpath -race -pgo merged.pprof
 
 littleauth: *.go go.*
 	$(go_build_cmd) -o $@

pgo.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"net/http"
+	_ "net/http/pprof" //#nosec: G108
+	"os"
+)
+
+func init() {
+	go func() {
+		if os.Getenv("COLLECT_METRICS") == "yes" {
+			//#nosec: G114
+			print(http.ListenAndServe("localhost:6060", nil), "\n")
+		}
+	}()
+}