get_probes lists some metrics twice

[wlach]

This script will generate a bunch of duplicate metrics:

from mozilla_schema_generator.probes import GleanProbe
from mozilla_schema_generator.glean_ping import GleanPing
repo = next((r for r in GleanPing.get_repos() if r["name"] == "burnham"))
glean_app = GleanPing(repo)
for (k, num) in dict(Counter([p.name for p in glean_app.get_probes()])).items():
    if num > 1:
        print(f"{k}: {num}")

As of this writing, I get this output:

app_build: 2
app_channel: 2
app_display_version: 2
architecture: 2
client_id: 2
device_manufacturer: 2
device_model: 2
first_run_date: 2
glean.baseline.duration: 2
glean.baseline.locale: 2
glean.error.invalid_label: 2
glean.error.invalid_value: 2
glean.error.preinit_tasks_timeout: 2
os: 2
os_version: 2

This is almost certainly the root cause of mozilla/lookml-generator#149