Check metrics-observer MSRV in CI

Author: djc

.github/workflows/ci.yml

@@ -48,7 +48,7 @@ jobs:
       # the rust-toolchain.toml file as metrics-observer requires 1.74 to build. See
       # https://github.com/metrics-rs/metrics/pull/505#discussion_r1724092556 for more information.
       matrix:
-        rust_version: ['stable', 'nightly', '1.71.1']
+        rust_version: ['stable', 'nightly', '1.74.0', '1.71.1']
         include:
           - rust_version: '1.71.1'
             exclude-packages: '--exclude metrics-observer'