Fix issue with `unsupported` metadata type for summary metric type - …

…see #120

perf: pool bytes.Buffer in WritePrometheus to reduce allocations (#115)

Use sync.Pool to reuse bytes.Buffer in (*Set).WritePrometheus,
eliminating repeated bytes.growSlice allocations.

Production pprof shows alloc_space dropped by 98.6% (12,380 MB → 170 MB)
and alloc_objects dropped by 96.6% (71.6M → 2.5M).

prefix vm_get_memory_info C func so it does not conflict with Prometh…

…eus one

Follow up on #107

Running tests on VM produced an error:

duplicate symbol '_get_memory_info' in:

/private/var/folders/ds/3kj5p3v17ll0hsyvq380ryvm0000gn/T/go-link-2626146643/000002.o

/private/var/folders/ds/3kj5p3v17ll0hsyvq380ryvm0000gn/T/go-link-2626146643/000015.o
ld: 1 duplicate symbols
clang: error: linker command failed with exit code 1 (use -v to see
invocation)

ok      github.com/VictoriaMetrics/VictoriaMetrics/lib/mergeset 1.820s

Add some process metrics for GOOS=darwin (#107)

Adds support for process metrics on MacOS\Darwin. I added only those
that do not require CGO.
- ~~process_start_time_seconds~~
- process_cpu_seconds_total
- process_virtual_memory_max_bytes
- process_virtual_memory_bytes
- process_resident_memory_bytes
- process_open_fds
- process_max_fds

Fixes #75

Inspired by
https://github.com/prometheus/client_golang/blob/main/prometheus/process_collector_darwin.go

WritePrometheus: reduce metricType() calls.