fix(beyla): Inject Beyla version into binary via ldflags

[pratik50]

Brief description of Pull Request

When Beyla is used as a library inside Alloy, the beyla_build_info metric was reporting version="unset" because Beyla's build-time version flags were never passed during Alloy's build process.

This PR injects the Beyla version into the Alloy binary via -ldflags at compile time, so the correct version is exposed via the beyla_build_info metric.

Pull Request Details

Alloy builds Beyla as a Go library dependency. Unlike standalone Beyla builds, Alloy's build system was not passing the ldflags needed to set pkg/buildinfo.Version, causing it to fall back to its default value of "unset".

The fix retrieves the Beyla version directly from go.mod using go list -m and injects it at build time.

Issue fixed by this Pull Request

Fixes grafana/beyla#2584
Fixes #4385

Changes are minimal and focused:

• Makefile — fetch Beyla version from go.mod and inject via ldflags
• common/metric.go — add Version field to parse the version label
• beyla_test.go — add test to verify version is not "unset"

PR Checklist

• Tests updated