Implement collector and basic CLI

.github/workflows/release.yaml

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+name: release
+on:
+  push:
+    tags:
+      - 'v*'
+
+jobs:
+  test:
+    name: Build and package
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: Setup Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: '~1.16.3'
+    - name: Run GoReleaser
+      uses: goreleaser/goreleaser-action@v2
+      with:
+        version: latest
+        args: release --rm-dist
+      env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.goreleaser.yml

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+project_name: vattenfall
+builds:
+  - id: vattenfall
+    binary: vattenfall
+    mod_timestamp: '{{ .CommitTimestamp }}'
+    flags:
+      - -trimpath
+    ldflags:
+      - -s
+      - -w
+      - -X main.version={{.Version}} -X main.commit={{.FullCommit}} -X main.date={{.CommitDate}} -X main.repository={{.GitURL}}
+    goos:
+      - windows
+      - darwin
+      - linux
+    goarch:
+      - amd64
+      - arm64
+      - arm
+    goarm:
+      - 7
+archives:
+  - id: vattenfall
+    builds:
+      - vattenfall
+    wrap_in_directory: true
+    files:
+      - LICENSE
+      - README.md
+    replacements:
+      darwin: macOS
+    format_overrides:
+      - goos: windows
+        format: zip

README.md

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+# Vattenfall
+
+This is a very basic Prometheus exporter for the Vattenfall electricity spot
+prices in Sweden. It'll export one metric, `energy_price_per_kwh` for each
+region:
+
+```
+# HELP energy_price_per_kwh Energy price per kWh for a region
+# TYPE energy_price_per_kwh gauge
+energy_price_per_kwh{country="SE",currency="SEK",region="SN1"} 0.4707
+energy_price_per_kwh{country="SE",currency="SEK",region="SN2"} 0.4707
+energy_price_per_kwh{country="SE",currency="SEK",region="SN3"} 0.4707
+energy_price_per_kwh{country="SE",currency="SEK",region="SN4"} 0.4707
+```
+
+Data is cached for 30min in memory to not hammer Vattenfall each time you
+scrape the collector (and their API is slow), but still catch any price
+adjustments that might occur.
+
+## Usage
+
+```
+-output.file string
+    write metrics to specified file (must have .prom extension)
+-output.http string
+    host:port to listen on for HTTP scrapes
+-region value
+    region to query for, SN1-4, can be passed multiple times
+```
+
+To run it as a Prometheus exporter that you can query over HTTP:
+
+```sh
+$ vattenfall -output.http=":9000" -region SN1 -region SN2 -region SN3 -region SN4
+```
+
+Please note that there's 2 endpoints `/metrics` which instruments the
+collector itself, and `/prices` with the pricing info.
+
+If you want to use it with the textfile collector, for example in an hourly cron:
+
+```sh
+$ vattenfall -output.file="/etc/prometheus/textfile/electricity.prom" -region SN1 -region SN2 -region SN3 -region SN4
+```
+
+Or to just get the values on the console:
+
+```sh
+$ vattenfall -region SN1 -region SN2 -region SN3 -region SN4
+```

data.go

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+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"sync"
+	"time"
+)
+
+const (
+	source = "https://www.vattenfall.se/api/price/spot/pricearea/%s/%s/%s"
+)
+
+type Data struct {
+	Timestamp time.Time
+	Region    string
+	Value     float64
+	Currency  string
+}
+
+var (
+	info      = map[string][]Data{}
+	lastFetch = map[string]time.Time{}
+	lock      = sync.RWMutex{}
+)
+
+func (d *Data) UnmarshalJSON(data []byte) error {
+	type internal struct {
+		Timestamp string  `json:"TimeStamp"`
+		Value     float64 `json:"Value"`
+		PriceArea string  `json:"PriceArea"`
+	}
+
+	var v internal
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	parsed, err := time.Parse("2006-01-02T15:04:05", v.Timestamp)
+	if err != nil {
+		return err
+	}
+
+	d.Timestamp = parsed
+	d.Region = v.PriceArea
+	d.Value = (v.Value * 100) / 10000
+	d.Currency = "SEK"
+
+	return nil
+}
+
+func fetch(date time.Time, region string) ([]Data, error) {
+	lock.RLock()
+	lf := lastFetch[region]
+	lock.RUnlock()
+
+	if lf.Add(30 * time.Minute).Before(date) {
+		b, err := fetchFromURL(date, region)
+		if err != nil {
+			return nil, err
+		}
+
+		data := []Data{}
+		err = json.Unmarshal(b, &data)
+		if err != nil {
+			return nil, err
+		}
+
+		lock.Lock()
+		lastFetch[region] = date
+		info[region] = data
+		lock.Unlock()
+
+		return data, nil
+	}
+
+	return info[region], nil
+}
+
+func fetchFromURL(date time.Time, region string) ([]byte, error) {
+	res := fmt.Sprintf(
+		source,
+		date.Format("2006-01-02"),
+		date.Format("2006-01-02"),
+		region,
+	)
+	resp, err := http.Get(res)
+	if err != nil {
+		return nil, fmt.Errorf("failed to fetch %s: %w", res, err)
+	}
+	defer func() {
+		io.Copy(ioutil.Discard, resp.Body)
+		resp.Body.Close()
+	}()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("expected 200 OK when fetching: %s, got: %d", res, resp.StatusCode)
+	}
+
+	data, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read response body: %w", err)
+	}
+
+	return data, nil
+}

go.mod

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+module github.com/daenney/vattenfall
+
+go 1.16
+
+require (
+	github.com/prometheus/client_golang v1.10.0
+	github.com/prometheus/common v0.18.0
+)