HTTP probe with `file_targets` can begin first iteration with 0 targets

[benner]

Describe the bug
HTTP probe with file_targets can begin first iteration with 0 targets

HEAD (933e248)

To Reproduce
Steps to reproduce the behavior:

1. Have relatively large targets.textpb file
2. Configure probe with file_path
3. Start Cloudprober with -debug
4. Watch logs

Additional context

% ./cloudprober -cloud_metadata none -logtostderr -config_file cloudprober.cfg -debug_log 
I0624 18:33:43.917872  219327 file.go:87] [cloudprober.test1-HTTPS] file_targets: Determined file format from file name: TEXTPB
I0624 18:33:43.917941  219327 prober.go:108] [cloudprober.global] Creating a HTTP probe: test1-HTTPS
I0624 18:33:43.917969  219327 http.go:238] [cloudprober.test1-HTTPS] Targets update interval: 1h0m0s
I0624 18:33:43.920664  219327 prometheus.go:177] [cloudprober.prometheus] Initialized prometheus exporter at the URL: /metrics
I0624 18:33:43.923160  219327 http.go:472] [cloudprober.test1-HTTPS] Probe(test1-HTTPS) got 0 targets
I0624 18:33:43.923174  219327 http.go:472] [cloudprober.test1-HTTPS] Probe(test1-HTTPS) got 0 targets
I0624 18:33:43.923196  219327 sysvars.go:185] [cloudprober.sysvars] 1624548823 labels=ptype=sysvars,probe=sysvars hostname="X" start_timestamp="1624548823" version="undefined"
I0624 18:33:49.213382  219327 file.go:196] [cloudprober.test1-HTTPS] file_targets(/tmp/targets.textpb): Read 1000000 resources, kept 289 after filtering
I0624 18:33:53.923683  219327 sysvars.go:198] [cloudprober.sysvars] 1624548833 labels=ptype=sysvars,probe=sysvars hostname="X" start_timestamp="1624548823" version="undefined"

Problem noticeable only with longer probe interval.

[manugarg]

Assigned it to the next milestone. Closing this issue now.