- allows custom tags to be added to prometheus metrics - Addresses GH issue #4021

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#### Type Of Change - [ ] Small bug fix (non-breaking change which fixes an issue) - [X] New feature (non-breaking change which adds new functionality) - [ ] Breaking change (fix or feature that would change existing functionality)

#### Checklist: <!-- Go over all points below, and after creating the PR, tick the checkboxes that apply --> - [X] PR should be backported to stable branches - [X] Tested changes locally - [X] Related to issue #4021

#### Description <!-- Describe your changes in detail --> This change adds a new parameter `xhttp_prom_tags`. It allows the user to append tags to the prometheus metrics. This makes those metrics a lot more useful when having multiple kamailio instances running on a single host.

This is done by 2 shared memory helper variables which contain the parameter in different formats. This way it is easy to print them out with minimal changes. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/4070

-- Commit Summary --

* xhttp_prom: new xhttp_prom_tags parameter

-- File Changes --

M src/modules/xhttp_prom/doc/xhttp_prom_admin.xml (25) M src/modules/xhttp_prom/prom.c (25) M src/modules/xhttp_prom/xhttp_prom.c (45) M src/modules/xhttp_prom/xhttp_prom.h (15)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/4070.patch https://github.com/kamailio/kamailio/pull/4070.diff