Failed to call mallctl with metrics and jemalloc (version 1) enabled

This issue has been migrated from [#10731](https://github.com/matrix-org/synapse/issues/10731).

---

### Description

My logs are full of the following warnings
```
2021-08-31 22:02:23,539 - synapse.metrics.jemalloc - 176 - WARNING - sentinel - Failed to read jemalloc stats.resident: Failed to call mallctl
2021-08-31 22:02:23,539 - synapse.metrics.jemalloc - 176 - WARNING - sentinel - Failed to read jemalloc stats.metadata: Failed to call mallctl
```

### Steps to reproduce

- enable jemalloc by setting the environment variable e.g. `LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.1`
- enable metrics collection

The server and metrics appear to still be working properly so I'm not sure what the impact of this warning is.

### Version information

- **Homeserver**: Other

If not matrix.org:

- **Version**: 1.41.1

- **Install method**: matrix.org Debian package repository

- **Platform**:

OS: Ubuntu 18.04.5 LTS
Virtualized: Yes

Start at the synapse.metrics.jemalloc entry point named in the warnings and reproduce with metrics enabled and LD_PRELOAD set to libjemalloc.so.1. Compare the handling of failed mallctl calls for stats.resident and stats.metadata; done means the warnings are addressed without disrupting the server or metrics collection.

Written by the indexing model from the issue text.