Add gcp.Meta function, equivalent to aws.EC2Meta

This commit adds a new namespace and function: `gcp.Meta`, which can be
used to look up values from the GCP Instance Metadata service.

An example usage:

```
echo '{{ gcp.Meta "id" }}' | gomplate
```

This also supports paths, so usage like this works:

```
echo '{{ gcp.Meta "network-interfaces/0/ip" }}' | gomplate
```

1 files changed, 36 insertions, 0 deletions

diff --git a/docs-src/content/functions/gcp.yml b/docs-src/content/functions/gcp.yml
new file mode 100644
index 00000000..036b1ecb
--- /dev/null
+++ b/docs-src/content/functions/gcp.yml
@@ -0,0 +1,36 @@
+ns: gcp
+preamble: |
+  The functions in the `gcp` namespace interface with various Google Cloud Platform
+  APIs to make it possible for a template to render differently based on the GCP
+  environment and metadata.
+
+  ### Configuring GCP
+
+  A number of environment variables can be used to control how gomplate communicates
+  with GCP APIs.
+
+  | Environment Variable | Description |
+  | -------------------- | ----------- |
+  | `GCP_META_ENDPOINT` | _(Default `http://metadata.google.internal`)_ Sets the base address of the instance metadata service. |
+  | `GCP_TIMEOUT` | _(Default `500`)_ Adjusts timeout for API requests, in milliseconds. |
+funcs:
+  - name: gcp.Meta
+    description: |
+      Queries GCP [Instance Metadata](https://cloud.google.com/compute/docs/storing-retrieving-metadata) for information.
+
+      For times when running outside GCP, or when the metadata API can't be reached, a `default` value can be provided.
+    pipeline: false
+    arguments:
+      - name: key
+        required: true
+        description: the metadata key to query
+      - name: default
+        required: false
+        description: the default value
+    examples:
+      - |
+        $ echo '{{gcp.Meta "id"}}' | gomplate
+        1334999446930701104
+      - |
+        $ echo '{{gcp.Meta "network-interfaces/0/ip"}}' | gomplate
+        10.128.0.23