feat(grafana): Add dashboard auto-downloader (#41665)

**Description**
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Please include a summary of the change and which issue is fixed. Please
also include relevant motivation and context. List any dependencies that
are required for this change.
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Currently, this chart allows provisioning of 3 predefined dashboards.

This is not ideal because:

- It is not flexible (only 3 statically defined in the chart dashboards
are supported)
- It bloats the chart unnecessarily
- Dashboards can get out of date quickly

To fix these shortcomings, I have added a curl init container that
executes a script which is generated during chart rendering based on the
values defined in the new map in the chart values
(`.Values.extraDashboards`). The map allows defining dashboards to be
provisioned both directly from `grafana.com` and via any URL that
returns a valid JSON Grafana dashboard file.

⚒️ Fixes  # <!--(issue)-->

**⚙️ Type of change**

- [x] ⚙️ Feature/App addition
- [ ] 🪛 Bugfix
- [ ] ⚠️ Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 🔃 Refactor of current code
- [ ] 📜 Documentation Changes

**🧪 How Has This Been Tested?**
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Please describe the tests that you ran to verify your changes. Provide
instructions so we can reproduce. Please also list any relevant details
for your test configuration
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- Rendered the chart manually with the values defined under
`.Values.extraDashboards`.
- Checked that the dashboards are present in Grafana after it starts

**📃 Notes:**

- The Grafana image technically already has `curl` as a command;
however, I chose to use a separate image just for the init container
because `curl` might be removed without notice from the Grafana image in
the future, and I do not want to depend on it being present.
- I have left the old way of deploying dashboards to prevent making this
a breaking change. After this is merged, I will open a new PR for
removing them (that will be marked breaking), and it can be merged when
appropriate.

**✔️ Checklist:**

- [x] ⚖️ My code follows the style guidelines of this project
- [x] 👀 I have performed a self-review of my own code
- [x] #️⃣ I have commented my code, particularly in hard-to-understand
areas
- [ ] 📄 I have made changes to the documentation
- [x] 🧪 I have added tests to this description that prove my fix is
effective or that my feature works
- [x] ⬆️ I increased versions for any altered app according to semantic
versioning
- [x] I made sure the title starts with `feat(chart-name):`,
`fix(chart-name):`, `chore(chart-name):`, `docs(chart-name):` or
`fix(docs):`

**➕ App addition**

If this PR is an app addition please make sure you have done the
following.

- [ ] 🖼️ I have added an icon in the Chart's root directory called
`icon.png`

---

_Please don't blindly check all the boxes. Read them and only check
those that apply.
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about and
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---------

Signed-off-by: Boemeltrein <130394941+Boemeltrein@users.noreply.github.com>
Signed-off-by: Kjeld Schouten <info@kjeldschouten.nl>
Co-authored-by: Boemeltrein <info@boemeltrein.nl>
Co-authored-by: Boemeltrein <130394941+Boemeltrein@users.noreply.github.com>
Co-authored-by: Kjeld Schouten <info@kjeldschouten.nl>

.github/renovate/main.json5

@@ -37,7 +37,8 @@
     "github>trueforge-org/truecharts//.github/renovate/special/customRules.json5",
     "github>trueforge-org/truecharts//.github/renovate/special/customVersioning.json5",
     "github>trueforge-org/truecharts//.github/renovate/special/customTalosClustertool.json5",
-    "github>trueforge-org/truecharts//.github/renovate/special/devcontainer.json5"
+    "github>trueforge-org/truecharts//.github/renovate/special/devcontainer.json5",
+    "github>trueforge-org/truecharts//.github/renovate/special/grafanadashboards.json5"
   ],
   // Main Renovate configuration that lists package rules for various paths
   "packageRules": [

.github/renovate/special/grafanadashboards.json5

@@ -0,0 +1,38 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "customDatasources": {
+    "grafana-dashboards": {
+      "defaultRegistryUrlTemplate": "https://grafana.com/api/dashboards/{{packageName}}",
+      "format": "json",
+      "transformTemplates": [
+        "{\"releases\":[{\"version\": $string(revision)}]}"
+      ]
+    }
+  },
+  "customManagers": [
+    {
+      "customType": "regex",
+      "description": ["Process Grafana dashboards"],
+      "fileMatch": [
+        "(^|/)kubernetes/.+\\.ya?ml(?:\\.j2)?$"
+      ],
+      "matchStrings": [
+        "depName=\"(?<depName>.*)\"\\n(?<indentation>\\s+)id: (?<packageName>\\d+)\\n.+revision: (?<currentValue>\\d+)"
+      ],
+      "autoReplaceStringTemplate": "depName=\"{{{depName}}}\"\n{{{indentation}}}id: {{{packageName}}}\n{{{indentation}}}revision: {{{newValue}}}",
+      "datasourceTemplate": "custom.grafana-dashboards",
+      "versioningTemplate": "regex:^(?<major>\\d+)$"
+    }
+  ],
+  "packageRules": [
+    {
+      "addLabels": ["renovate/grafana-dashboard"],
+      "matchDatasources": ["custom.grafana-dashboards"],
+      "matchUpdateTypes": ["major"],
+      "semanticCommitType": "chore",
+      "semanticCommitScope": "grafana-dashboards",
+      "commitMessageTopic": "dashboard {{depName}}",
+      "commitMessageExtra": "( {{currentVersion}} → {{newVersion}} )"
+    }
+  ]
+}