Add Databricks billing plugin

manifest

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 datadog
 mongodb-atlas
 openai
+databricks

pkg/plugins/databricks/README.md

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+# Databricks OpenCost Plugin
+
+The Databricks plugin imports Databricks account billable usage logs into OpenCost Custom Costs.
+
+It uses the account-level billable usage download API:
+
+```text
+GET /api/2.0/accounts/{account_id}/usage/download
+```
+
+The API returns CSV usage logs for a requested month range. The plugin downloads each required month once per OpenCost request, filters the records into daily windows, and converts usage rows into Custom Costs.
+
+## Configuration
+
+Create a plugin config JSON file:
+
+```json
+{
+  "account_id": "<databricks-account-id>",
+  "token": "<databricks-account-token>",
+  "sku_unit_prices": {
+    "STANDARD_ALL_PURPOSE_COMPUTE": 0.55
+  },
+  "default_unit_price": 0,
+  "log_level": "info"
+}
+```
+
+Optional fields:
+
+```json
+{
+  "account_api_base_url": "https://accounts.cloud.databricks.com",
+  "include_personal_data": false
+}
+```
+
+Databricks usage downloads provide usage quantity by SKU, while actual pricing varies by account contract. Configure `sku_unit_prices` for known per-unit prices and optionally set `default_unit_price` as a fallback. Rows without a configured price are reported with usage quantity and zero cost unless `default_unit_price` is set.
+
+## Cost Mapping
+
+- `usage_quantity` and `usage_unit` -> usage quantity and unit
+- configured SKU unit price multiplied by `usage_quantity` -> billed and list cost
+- `account_id` -> account name and account ID
+- `workspace_id` -> sub-account ID
+- `billing_origin_product` or `usage_type` -> service and resource type
+- `sku_name` -> SKU ID
+- `record_id` -> provider ID
+
+Only daily resolution is supported. The plugin requests monthly CSV exports because Databricks exposes the account billable usage download API with `start_month` and `end_month` parameters.