Otel Metrics payload can be too long

[GeorgeHahn]

CI failures have been observed with the following payload length error:

 ---- payload::opentelemetry_metric::test::payload_not_exceed_max_bytes stdout ----
thread 'payload::opentelemetry_metric::test::payload_not_exceed_max_bytes' panicked at 'max len: 250, actual: 251', src/payload/opentelemetry_metric.rs:236:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'payload::opentelemetry_metric::test::payload_not_exceed_max_bytes' panicked at 'max len: 250, actual: 251', src/payload/opentelemetry_metric.rs:236:13
thread 'payload::opentelemetry_metric::test::payload_not_exceed_max_bytes' panicked at 'max len: 250, actual: 251', src/payload/opentelemetry_metric.rs:236:13
thread 'payload::opentelemetry_metric::test::payload_not_exceed_max_bytes' panicked at 'max len: 250, actual: 251', src/payload/opentelemetry_metric.rs:236:13
proptest: Saving this and future failures in /Users/runner/work/lading/lading/proptest-regressions/payload/opentelemetry_metric.txt
proptest: If this test was run on a CI system, you may wish to add the following line to your copy of the file. (You may need to create it.)
cc 5ada42712d32e65e5b5292cbe437a53495db6a483c0f612a24ea5d1846d2e787
thread 'payload::opentelemetry_metric::test::payload_not_exceed_max_bytes' panicked at 'Test failed: max len: 250, actual: 251; minimal failing input: seed = 16556747614468308026, max_bytes = 250
	successes: 111
	local rejects: 0
	global rejects: 0
', src/payload/opentelemetry_metric.rs:227:5