static-exporter: Update httpd.conf file to include headers (#1361)

* static-exporter: Update httpd.conf file to include headers

Prometheus v3.0.0 has a stricter requirements on the headers that are
returned from exporters. The static-exporter does not set any headers
and causes scrapes to fail with the following error:

```
non-compliant scrape target sending blank Content-Type and no fallback_scrape_protocol specified for target ````
```

This adds an httpd.conf file to mount to the container that includes a
directive to add a header that solves this problem. To generate the
httpd.conf file, I ran the following command:

`docker run --rm httpd:2.4 cat /usr/local/apache2/conf/httpd.conf > my-httpd.conf`

* Mount the httpdConfig to the filesystem

* Include withSubPath mixin volumeMount mixin

* Move httpConfig setup to a method so it's opt in

* Add reference to name that can be called elsewhere

* Update withHttpConfig() to accept a config that can be patched

* Update configMap to have a prefix with the name and a suffix foir -httpd-config

* Update README.md for static-exporter to add explicit instructions to update the httpd.conf

* Update static-exporter/httpd.conf

Co-authored-by: Jeroen Op 't Eynde <[email protected]>

---------

Co-authored-by: Zach Leslie <[email protected]>
Co-authored-by: Jeroen Op 't Eynde <[email protected]>

Author: 3 people

static-exporter/README.md

@@ -43,3 +43,22 @@ local static_exporter = import 'github.com/grafana/jsonnet-libs/static-expoter/m
     ]),
 }
 ```
+
+## Updating httpd.conf
+
+There is a default httpd.conf that was added to this library.
+It was generated by running the following:
+
+```
+docker run --rm httpd:2.4 cat /usr/local/apache2/conf/httpd.conf httpd.conf
+```
+
+If there is a downstream change that requires updating this config file, run the above command and then add the following snippet to the `<Directory "/usr/local/apache2/htdocs"` block:
+
+```
+  <IfModule mod_headers.c>
+        Header set Content-Type: "text/plain; version=0.0.4"
+    </IfModule>
+```
+
+This change adds a Header to the requests that enables Prometheus 3.x to scrape the static exporter.