feat(stackdriver_exporter): Add ErrorLogger for promhttp (#277)

* feat(stackdriver_exporter): Add ErrorLogger for promhttp

I had recently experienced #103 and #166 in production and it took quite
some time to recognize there was a problem with `stackdriver_exporter`
because nothing was logged out to indiciate problems gathering metrics.
From my perspective, the pod was healthy and online and I could
curl `/metrics` to get results. Grafana Agent however was getting errors
when scraping, specifically errors like so:

```
 [from Gatherer #2] collected metric "stackdriver_gce_instance_compute_googleapis_com_instance_disk_write_bytes_count" { label:{name:"device_name"
value:"REDACTED_FOR_SECURITY"} label:{name:"device_type"  value:"permanent"} label:{name:"instance_id" value:"2924941021702260446"} label:{name:"instance_name"  value:"REDACTED_FOR_SECURITY"} label:{name:"project_id" value:"REDACTED_FOR_SECURITY"}  label:{name:"storage_type" value:"pd-ssd"} label:{name:"unit" value:"By"} label:{name:"zone" value:"us-central1-a"}
counter:{value:0} timestamp_ms:1698871080000} was collected before with the same name and label values
```

To help identify the root cause I've added the ability to opt into
logging out errors that come from the handler. Specifically,
I've created the struct `customPromErrorLogger` that implements the `promhttp.http.Logger` interface.
There is a new flag: `monitoring.enable-promhttp-custom-logger` which if it is set to true, then
we create an instance of `customPromErrorLogger` and use it as the value for ErrorLogger
in `promhttp.Handler{}`. Otherwise, `stackdriver_exporter` works as it
did before and does not log out errors collectoing metrics.

- refs #103, #166

---------

Signed-off-by: pokom <[email protected]>

Author: Pokom

stackdriver_exporter.go

@@ -15,6 +15,7 @@ package main
 
 import (
 	"fmt"
+	stdlog "log"
 	"net/http"
 	"os"
 	"strings"
@@ -238,9 +239,9 @@ func (h *handler) innerHandler(filters map[string]bool) http.Handler {
 			registry,
 		}
 	}
-
+	opts := promhttp.HandlerOpts{ErrorLog: stdlog.New(log.NewStdlibAdapter(level.Error(h.logger)), "", 0)}
 	// Delegate http serving to Prometheus client library, which will call collector.Collect.
-	return promhttp.HandlerFor(gatherers, promhttp.HandlerOpts{})
+	return promhttp.HandlerFor(gatherers, opts)
 }
 
 // filterMetricTypePrefixes filters the initial list of metric type prefixes, with the ones coming from an individual