Introduce built-in Prometheus exporter to the Postgres extension (#12591)

Currently, the exporter exposes the same LFC metrics that are exposed by
the "autoscaling" sql_exporter in the docker image. With this, we can
remove the dedicated sql_exporter instance. (Actually doing the removal
is left as a TODO until this is rolled out to production and we have
changed autoscaling-agent to fetch the metrics from this new endpoint.)

The exporter runs as a Postgres background worker process. This is
extracted from the Rust communicator rewrite project, which will use the
same worker process for much more, to handle the communications with the
pageservers. For now, though, it merely handles the metrics requests.

In the future, we will add more metrics, and perhaps even APIs to
control the running Postgres instance.

The exporter listens on a Unix Domain socket within the Postgres data
directory. A Unix Domain socket is a bit unconventional, but it has some
advantages:

- Permissions are taken care of. Only processes that can access the data
directory, and therefore already have full access to the running
Postgres instance, can connect to it.

- No need to allocate and manage a new port number for the listener

It has some downsides too: it's not immediately accessible from the
outside world, and the functions to work with Unix Domain sockets are
more low-level than TCP sockets (see the symlink hack in
`postgres_metrics_client.rs`, for example).

To expose the metrics from the local Unix Domain Socket to the
autoscaling agent, introduce a new '/autoscaling_metrics' endpoint in
the compute_ctl's HTTP server. Currently it merely forwards the request
to the Postgres instance, but we could add rate limiting and access
control there in the future.

---------

Co-authored-by: Conrad Ludgate <conrad@neon.tech>

poetry.lock

@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 2.1.1 and should not be changed by hand.
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 [[package]]
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@@ -3068,6 +3068,21 @@ urllib3 = ">=1.21.1,<3"
 socks = ["PySocks (>=1.5.6,!=1.5.7)"]
 use-chardet-on-py3 = ["chardet (>=3.0.2,<6)"]
 
+[[package]]
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+description = "Use requests to talk HTTP via a UNIX domain socket"
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+python-versions = ">=3.9"
+groups = ["main"]
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+    {file = "requests_unixsocket-0.4.1.tar.gz", hash = "sha256:b2596158c356ecee68d27ba469a52211230ac6fb0cde8b66afb19f0ed47a1995"},
+]
+
+[package.dependencies]
+requests = ">=1.1"
+
 [[package]]
 name = "responses"
 version = "0.25.3"
@@ -3844,4 +3859,4 @@ cffi = ["cffi (>=1.11)"]
 [metadata]
 lock-version = "2.1"
 python-versions = "^3.11"
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