Use official prometheus_client version 0.21.0, once released

[andy-maier]

PR #558 vendorized prometheus_client version 0.20.0.post1 from my fork.

This ticket is to remove the vendorizing again, and to use the officially released version that contains these two PRs:

• Removed CBC ciphers to address CVE-2013-0169 (LUCKY13) prometheus/client_python#1051
• Reject invalid HTTP methods and resources prometheus/client_python#1019 - released in 0.21.0