feat: add additional clause for Telem.client_join

[J0]

What kind of change does this PR introduce?

Fix #214.

Omitted a test on this one since it seems like we don't test telem.ex

[J0]

Actually on further thought maybe we should write a test. Will do so tomorrow

[J0]

Found it a little hard to write a test so I tested it manually using telnet

It now returns

16:32:42.351 [error] Client startup message error: :bad_startup_payload file=lib/supavisor/client_handler.ex line=149 pid=<0.1056.0>
16:32:42.351 [warning] client_join is called with a mismatched id: nil file=lib/supavisor/monitoring/telem.ex line=61 pid=<0.1056.0>

[J0]

Hey @abc3

Sorry to bother you again - just going through list of existing open PRs to close them out. Any chance I could get a review on this again when free?

Here's how it was tested:

1. run docker compose -f docker-compose-db.yml up
2. run make dev
3. run telnet localhost 6543 and then enter a foreign character (e.g. q). See the error message (as below)

Lmk if there's more I should add or more cases that we should handle