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

[Pokom]

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

[jjo]

1st pass:

• Please add some bits from this PR description into README.md, especially the httpd.conf should be updated, in case the (hardcoded) image changes
• nit: in above instructions, let's also use precisely the same image, i.e. docker run --rm httpd:2.4-alpine ...