nix: add process monitoring for loadtests

Closes https://github.com/PostgREST/postgrest/issues/4107.

Adds two python scripts:

- monitor_pid.py: monitors the postgrest process each second
  until it exits, then outputs a csv with the results. The nix wrappers
  use the `loadtest` dir for the output.
- merge_monitor_result.py: receives a list of csvs and merges them into
  a single markdown table. The nix wrappers use the `loadtest/*.csv`
  files for the input.

The nix `postgrest-with-pgrst` and `postgrest-loadtest-report` commands
use these scripts to add monitoring for `postgrest-loadtest` and
`postgrest-loadtest-against`.

nix/tools/merge_monitor_result.py

@@ -0,0 +1,28 @@
+import os
+import sys
+import pandas as pd
+
+KEY = "Elapsed seconds"
+
+merged = None
+
+paths = [p.strip() for p in sys.stdin.read().split() if p.strip()]
+
+for csv_path in paths:
+    # pfix is prefix (variable shortened to pass linter)
+    pfix = os.path.splitext(os.path.basename(csv_path))[0]
+
+    df = pd.read_csv(csv_path)
+
+    if KEY not in df.columns:
+        sys.exit(f"{csv_path} is missing the {KEY} column")
+
+    # add prefix to every metric column
+    df = df.rename(columns={c: f"{pfix} {c}" for c in df.columns if c != KEY})
+
+    # outer join so missing rows appear
+    merged = df if merged is None else merged.merge(df, on=KEY, how="outer")
+
+# replace nan with empty string
+merged = merged.fillna("")
+merged.to_markdown(sys.stdout, index=False, tablefmt="github")