feat: set prometheus cont type

[btlghrants]

Description

Specifies content-type header on outgoing /metrics endpoint responses.

Related Issue

Fixes #1500

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Unit, Journey, E2E Tests, docs, adr added or updated as needed
• Contributor Guide Steps followed

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.12%. Comparing base (afdc893) to head (fb47886).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib/controller/index.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1501      +/-   ##
==========================================
- Coverage   79.17%   79.12%   -0.05%     
==========================================
  Files          39       39              
  Lines        1719     1720       +1     
  Branches      412      404       -8     
==========================================
  Hits         1361     1361              
- Misses        329      330       +1     
  Partials       29       29              

Files with missing lines	Coverage Δ
src/lib/controller/index.ts	10.74% <0.00%> (-0.09%)	⬇️

[cmwylie19]

this will work with Prom 3.0