perf/lua: add context section to test output

Google Benchmark output format contains a section "context" that
describes useful information about test environment.

Google Benchmark output format has been supported in Lua
microbenchmarks in commit 3110ef9a
("perf: introduce benchmark.lua helper module"). However, produced
output contains test results only and section "context" is missed.
The patch add a section "context" with the following fields:
date, load average, hostname, tarantool's version, build flags
and a name of build target.

```
$ tarantool uri_escape_unescape.lua --output=res.json --output_format=json
$ jq ".context" res.json
{
  "build_target": "Linux-x86_64-RelWithDebInfo",
  "host_name": "pony",
  "date": "2024-07-04 19:09:11",
  "tarantool_version": "3.2.0-entrypoint-114-g9e5dca29ad",
  "build_flags": " -fexceptions -funwind-tables -fasynchronous-unwind-tables -fno-common -msse2 -Wformat -Wformat-security -Werror=format-security -fstack-protector-strong -fPIC -fmacro-prefix-map=/home/sergeyb/sources/MRG/tarantool=. -std=c11 -Wall -Wextra -Wno-gnu-alignof-expression -Wno-cast-function-type -O2 -g -DNDEBUG -ggdb -O2 ",
  "load_avg": [
    "0.76",
    "0.74",
    "0.63"
  ]
}
```

NO_CHANGELOG=perf
NO_DOC=perf
NO_TEST=perf

perf/lua/benchmark.lua

@@ -34,6 +34,8 @@
 local json = require('json')
 local fio = require('fio')
 local argparse = require('internal.argparse')
+local tarantool = require('tarantool')
+local datetime = require('datetime')
 
 local M = {}
 
@@ -45,7 +47,10 @@ local function format_report(bench)
         -- The output should have the same format as the Google
         -- Benchmark JSON output format:
         -- https://github.com/google/benchmark/blob/main/docs/user_guide.md
-        report = json.encode({benchmarks = results})
+        report = json.encode({
+            benchmarks = results,
+            context = bench.context,
+        })
     else
         assert(output_format == 'console', 'unknown output format')
         for _, res in ipairs(results) do
@@ -119,6 +124,42 @@ function M.argparse(arg, argtable, custom_help)
     return params
 end
 
+local function load_average()
+    local path = '/proc/loadavg'
+    local fh = assert(fio.open(path, {'O_RDONLY'}))
+    local loadavg_buf = fh:read(1024)
+    fh:close()
+    -- Format is '0.89 0.66 0.80 2/1576 1203510'.
+    local loadavg_re = '(%d+.%d+) (%d+.%d+) (%d+.%d+)'
+
+    return { string.match(loadavg_buf, loadavg_re) }
+end
+
+-- The function return a ISO 8061 formatted datetime.
+--
+-- Google Benchmark reports contains a date in ISO 8061 format,
+-- example: 2013-07-01T17:55:13-07:00.
+-- See an implementation of `LocalDateTimeString()` in
+-- Google Benchmark library source code [1].
+--
+-- 1. https://github.com/google/benchmark/blob/65668db27365d29ca4890cb2102e81acb6585b43/src/timers.cc#L209
+-- @return e.g. 2024-07-18T10:06:59+0000
+local function iso_8061_timestamp()
+    local dt = os.date('*t')
+    return datetime.new(dt):format('%Y-%m-%dT%H:%M:%S%z')
+end
+
+local function perf_context()
+    return {
+        build_flags = tarantool.build.flags,
+        build_target = tarantool.build.target,
+        date = iso_8061_timestamp(),
+        host_name = io.popen('hostname'):read(),
+        load_avg = load_average(),
+        tarantool_version = tarantool.version,
+    }
+end
+
 function M.new(opts)
     assert(type(opts) == 'table', 'given argument should be a table')
     local output_format = opts.output_format or 'console'
@@ -126,6 +167,7 @@ function M.new(opts)
         output = opts.output,
         output_format = output_format:lower(),
         results = {},
+        context = perf_context(),
     }, {__index = {
         add_result = add_result,
         dump_results = dump_results,